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  • 1995-1999  (433)
  • 1975-1979
  • 1920-1924
  • 1996  (433)
  • Computational Chemistry and Molecular Modeling  (433)
  • 101
    Electronic Resource
    Electronic Resource
    Modification of the local self-consistent field method for modeling surface reactivity of covalent solids (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1525-1536 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The local self-consistent field (LSCF) method which allows full SCF computations on a fragment of very long molecular systems represented by a classical force field has been adapted to the description of nonmetallic crystals. The periodicity of the network is achieved by modifying self-consistently the basic parameters of the classical subsystem (charges, geometric parameters) along the SCF iterative scheme. The method is tested on α-cristobalite. The parametrization of the quantum classical junction, achieved by localized bond orbitals has been performed with the help of a fragment located in the bulk. The stability of the method with respect to the size of the fragment and the size of the crystalline sample appears to be very good. The properties of fully hydroxylated (010) surface are corrected described. Modeling of water adsorption on the ideal surface as well as on two kinds of surface defects gives rise to very reasonable results with an absorption energies of ca 10 kcal/mol which are close to the upper limit of the experimental data. This preliminary study appears to be quite encouraging regarding the possibilities of using this method, which can be considered as an extension of the embedded cluster approach to covalent solids. Many applications to surface chemical reactivity studies can be imagined. © 1996 John Wiley & Sons, Inc.
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  • 102
    Electronic Resource
    Electronic Resource
    Computational studies of less common fullerene-related species (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1567-1576 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The contribution reports results of ongoing computations of various cage structures differing from the well-known buckminsterfullerene in the number of carbon atoms (C80) in their coordination or in the types of rings involved (C59). The computations have supplied a complete description of the seven isolated-pentagon-rule (IPR) isomers of C80 (i.e., a system not yet observed) at the semiempirical AM1 and SAM1 levels; their energetics was also evaluated at ab initio Hartree-Fock level in small basis sets. The ground-state structure of the system possesses a D5d symmetry, but at supposed synthetic conditions a structure of D2 symmetry is the most populated, i.e. temperature represents an important factor in the stability relationships. One of the IPR isomers exhibits an Ih topological symmetry, but it undergoes a Jahn-Teller distortion and its symmetry is actually reduced to D2. The C59 serves as an example of odd-numbered carbon clusters, and new rules are reported for the pentagon/hexagon pattern in the odd clusters, yielding a variable number of the pentagons. Moreover, other rings are considered and the computed ground state of the system in fact contains an eight-membered ring (while the next lowest species exhibits a nine-membered ring). Their relative stabilities are not very sensitive to temperature. Altogether 19 isomers of C59 are treated at the AM1 level. In contrast to even-numbered fullerenes, with odd fullerenes the cages cannot be built from three-coordinated carbon atoms only. Hence, we have to allow for two- or even four-coordinated atoms. For example, if we consider one two-coordinated carbon atom (and the rest three-coordinated) the number of pentagons drops to 10. If we allow for just one four-coordinated atom, the number of pentagons changes to 14. Let the number of two-coordinated carbon atoms be p, and the number of four-coordinated q. If we still allow only for pentagons and hexagons, it holds for the number of pentagons n5 = -2p + 2q + 12. © 1996 John Wiley & Sons, Inc.
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  • 103
    Electronic Resource
    Electronic Resource
    Comparisons of the theoretical calculation of nitrogen clusters by semiempirical MO method (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1709-1718 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Various nitrogen clusters, Nx, are selected for the present theoretical study. The number of nitrogen atoms chosen in this work varies from x = 8 to x = 32. PM3, which is known as one of the best semiempirical methods, is selected for the self-consistent molecular orbital calculations. The geometrical optimization, vibrational frequencies, and thermochemical computations are all involved for various types of molecular nitrogen clusters. The results show that all Nx's belong to the category of stable high-energy compounds. Comparison of average bond energy and delocalization energy of all cases reveals that N20(Ih symmetry) is the most stable molecule among all the nitrogen clusters studied. In addition, our results show five-membered rings are the most favored in the structures of nitrogen clusters (Nx). © 1996 John Wiley & Sons, Inc.
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  • 104
    Electronic Resource
    Electronic Resource
    Introduction (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1213-1213 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560600703
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  • 105
    Electronic Resource
    Electronic Resource
    Ab initio modeling of the interaction between guanine—cytosine base pair and mustard alkylating agents (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1745-1764 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Interaction between the guanine-cytosine base pair and the episulfonium form of sulfur mustard (HD+) or the aziridinium form of nitrogen mustard (HN2+) was studied using the ab initio LCAO-MO method at the HF/6-31G level. The alkylation mechanism on the guanine N7 was analyzed by using a supermolecular modeling. Our stereostructural results associated with the molecular electrostatic potentials and highest occupied/lowest unoccupied molecular orbital (HOMO(SINGLEBOND)LUMO) properties show that in vacuum the alkylation of the N7 of guanine by HD+ in the agressive episulfonium form is a direct process without a transition state and of which the pathway is determined. Our study shows that interaction of guanine with the aziridinium form of HN2+ necessitates a transition state for the N7 alkylation route. When the N7 guanine alkylation by HD+ or HN2+ is achieved, about half of a positive charge moves from the alkylator toward the guanine in both cases. © 1996 John Wiley & Sons, Inc.
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  • 106
    Electronic Resource
    Electronic Resource
    Quantitative measures of molecular similarity: Methods to analyze transition-state analogs for enzymatic reactions (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1797-1804 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A formalism is presented for quantifying the similarity between any two molecules. The chemical descriptor used for comparison is the molecular electrostatic potential at the van der Waals surface. Thus, both the spatial properties of a molecule and its chemical features are captured in this approach. For molecules that are geometrically alike, the most useful similarity measure stems from orienting the two species so that their physical surfaces are aligned as well as possible, without regard to chemical patterns. After this alignment is achieved, a single measure sensitive to the spatial distribution of the electrostatic potential is used to rank the electronic similarity. Molecular similarity measures are applied to the enzyme systems AMP deaminase and AMP nucleosidase in order to understand quantitatively why their respective transition-state inhibitors bind more tightly than do their substrates. © 1996 John Wiley & Sons, Inc.
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  • 107
    Electronic Resource
    Electronic Resource
    Enzymatic transition states and inhibitor design from principles of classical and quantum chemistry (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1805-1813 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A procedure is described which leads to experimentally based models for the transition-state structures of enzyme-catalyzed reactions. Substrates for an enzymic reaction are synthesized with isotopically enriched atoms at every position in which bonding changes are anticipated at the enzyme-enforced transition state. Kinetic isotope effects are measured for each atomic substitution and corrected for diminution of the isotope effects from nonchemical steps of the enzymic mechanism. A truncated geometric model of the transition-state structure is fitted to the kinetic isotope effects using bond-energy bond-order vibrational analysis. Full molecularity is restored to the transition state while maintaining the geometry of the bonds which define the transition state. Electronic wave functions are calculated for the substrate and the transition-state molecules. The molecular electrostatic potential energies are defined for the van der Waal surfaces of substrate and transition state and displayed in numerical and color-coded constructs. The electronic differences between substrate and transition state reveal characteristics of the transition state which permits the extraordinary binding affinity of enzyme-transition state interactions. The information has been used to characterize several enzymatic transition states and to design powerfully inhibitory transition-state analogues. Enzymatic examples are provided for the reactions catalyzed by AMP deaminase, nucleoside hydrolase, purine nucleoside phosphorylase, and for several bacterial toxins. The results demonstrate that the combination of experimental, classical, and quantum chemistry approaches is capable of providing reliable transition-state structures and sufficient information to permit the design of transition-state inhibitors. © 1996 John Wiley & Sons, Inc.
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  • 108
    Electronic Resource
    Electronic Resource
    Bond profiles for cuboctahedron and twist cuboctahedron (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1851-1863 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using the information on the interatomic separations in a molecule, one can construct structural invariants that are the components of a molecular profile. The entries in the profile are derived by averaging different powers of the interatomic separations, suitably normalized so that the increasing powers do not dominate the sequence. Although only a few hundreds of structures have been so analyzed, no two different chemical structures were found to be characterized by the same sequences. A critical test for the conjecture that molecular profiles are unique is to consider structurally closely related systems that are very similar and have several similar properties. In this contribution we investigated the cuboctahedron and the accompanying polyhedron obtained by rotating half of the cuboctahedron against the other half, resulting in the so-called twist cutoctahedron. We show that even this pair of closely related structures has different profiles. We have also examined the generalized molecular profiles obtained by inserting n additional points along each edge of the polyhedra. The convergence of the profiles as n increases is discussed. It appears thus that these generalized molecular profiles, called line profiles or bond profiles, are likely to lead to a unique characterization of structures in which not only the geometry of atoms is recorded but also the geometry of the connectivity of the structure. © 1996 John Wiley & Sons, Inc.
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  • 109
    Electronic Resource
    Electronic Resource
    Structural homeomorphism between the electron density and the virial field (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 183-198 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The virial field V(r) is defined by the local statement of the quantum mechanical virial theorem, as the trace of the Schrödinger stress tensor. This field defines the electronic potential energy density of an electron at r and integrates to minus twice the electronic kinetic energy. It is the most short-ranged description possible of the local electronic potential energy and it exhibits the same transferable behavior over bounded regions of real space (corresponding to the functional groups of chemistry) as does ρ(r). This article establishes a structural homeomorphism between -V(r) and ρ(r), showing that the two fields are homeomorphic over all of the nuclear configuration space. The stable or unstable structure defined by the gradient vector field Δρ(r; X) for any configuration X of the nuclei can be placed in a one-to-one correspondence with a structure defined by the field -ΔV(r; X′). In particular, a molecular graph for ρ(r) defining a molecular structure is mirrored by a corresponding virial graph for V(r) and the lines of maximum density linking bonded nuclei in the former field are matched by a set of lines of maximally negative potential energy density in the latter. The homeomorphism is also geometrically faithful, an equilibrium geometry in general, exhibiting equivalent structures in the two fields. The demonstration that the virial field, whose integrated value equals twice the total energy, is essentially just a locally scaled version of the electron density is suggestive of possible new approaches in density functional theory. © 1996 John Wiley & Sons, Inc.
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  • 110
    Electronic Resource
    Electronic Resource
    Calculation of bond dissociation energies of diatomic molecules using bond function basis sets with counterpoise corrections (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 207-212 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bond function basis sets combined with the counterpoise procedure are used to calculate the molecular dissociation energies De of 24 diatomic molecules and ions. The calculated values of De are compared to those without bond functions and/or counterpoise corrections. The equilibrium bond lengths re and harmonic frequencies οe are also calculated for a few selected molecules. The calculations at the fourth-order Møller-Plesset approximation (MP4) have consistently recovered about 95-99% of the experimental values for De; compared to as low as 75% without use of bond functions. The calculated values of re are typically 0.01 Å larger than the experimental values, and the calculated values of οe are over 95% of the experimental values. © 1996 John Wiley & Sons, Inc.
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  • 111
    Electronic Resource
    Electronic Resource
    Study of locally dense and locally saturated basis sets in localized molecular orbital calculations of nuclear shielding: Ab initio LORG calculations for 13C and 17O in norbornenone (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 199-205 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This study demonstrates the use of uneven atomic basis sets for ab initio calculations of NMR shielding in the localized orbital/local origin (LORG) approach with norbornenone as the test case. We distinguish between locally dense sets (extended basis on target atom only) and locally saturated sets (extended bases on target atom and atoms in its first bonding sphere), using 6-311G** and 6-31G sets to describe the high and low level of function sets. It is shown that the use of these uneven sets can simulate high basis set level calculations of shieldings for 1H and for all the 13C nuclei in this molecule and, hence, allows quite accurate ab initio calculations of shielding properties of these nuclei in large molecules using relatively modest computational facilities. The shielding of the double-bonded 17O nucleus is apparently sensitive to basis-set quality beyond the first bonding sphere. © 1996 John Wiley & Sons, Inc.
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  • 112
    Electronic Resource
    Electronic Resource
    Density functional calculations of difluorodiazete structures with Gaussian-orbital-type approach (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 213-217 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The comparison of local nonlocal and hybrid DFT methods with RHF, MP2, CCSD, and CPF ab initio methods in generating geometries and relative energies of cis- and trans-difluorodiazete, SVWN, BLYP, BP86, BECKE3LYP, and BECKE3P86 DFT methods with 6-311 + g(2d) and 6-311 + + g(3df) basis sets. The geometries generated with RHF ab initio models are quite different from experimental values and energy evaluation prefers the wrong isomer. The hybrid methods give superior geometries while energies evaluated with nonlocal DFT methods are better than the one obtained with MP2 or CCDS ab initio methods. The results suggest DFT as the method of choice of studying similar systems. © 1996 John Wiley & Sons, Inc.
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  • 113
    Electronic Resource
    Electronic Resource
    Sturmian basis sets in momentum space (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 401-411 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The properties of Sturmian basis sets in d-dimensional direct space and d-dimensional momentum space are reviewed, as well as the relationship between hydrogenlike Sturmians and hyperspherical harmonics. The kernel of the reciprocal-space Schrödinger equation is expanded in terms of Strumian basis sets. This expansion allows Shibuya and Wulfman's treatment of many-center Coulomb potentials to be extended to many-center potentials of a general form, and the method is also extended to the calculation of crystal orbitals and band structures. © 1996 John Wiley & Sons, Inc.
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  • 114
    Electronic Resource
    Electronic Resource
    The extended Koopmans' theorem Fock operator and the generalized overlap amplitude one-electron operator (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 391-399 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The wave function of a system may be expanded in terms of eigenfunctions of the N -1 electron Hamiltonian times one-particle functions known as generalized overlap amplitudes (GOAS). The one-electron operator whose eigenfunctions are the GOAS is presented, without using an energy-dependent term as in the one-particle Green function or propagator approach. It is shown that this operator and the extended Koopmans' theorem (EKT) one-electron operator are of similar form, but perform complementary roles. The GOA operator begins with one-electron densities and total energies of N -1 electron states to generate the two-matrix and total energy of an N-electron state. The EKT operator begins with the two-matrix of an N-electron state to generate one-electron densities and ionization potentials (or approximations thereto) for N -1 electron states. However, whereas the EKT orbitals must be linearly independent, no such restriction applies to the GOAS. © 1996 John Wiley & Sons, Inc.
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  • 115
    Electronic Resource
    Electronic Resource
    Quantum mechanical models in catalysis (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 413-422 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We applied general quantum mechanical ideas in order to establish the form of the many-electron wave functions suitable for analysis of catalytic processes. This led us to the conclusion that the relevant wave functions for the electrons of the catalytic complexes must be taken as superpositions of the antisymmetrized products of the wave functions of electrons in excited and ionized states of the catalyst and reactants. With use of the trial wave function for the electrons of the catalytic complex in such a form, it becomes possible to construct model potential energy surfaces of catalytic reactions as a superposition of the potential energy surfaces of the reactants in different electronic states. We formulate the criteria which when satisfied make it possible to implement a catalytic version of a desired chemical transformation. We also propose an approach to the explanation of the frequently observed correlations between the catalytic activity and other physical properties of a catalyst. © 1996 John Wiley & Sons, Inc.
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  • 116
    Electronic Resource
    Electronic Resource
    New formulation of the analytical electronic energy and the regional density functional theory (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 423-428 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A certain analytical model of E is proposed to satisfy the prerequisite exact density functional theory (DFT) formulas such as μ = ∂E/∂N = -(I + A)/2 and 2η = ∂2E/∂N2 = I -A. © 1996 John Wiley & Sons, Inc.
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  • 117
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    On the fragmentation dynamics of NH3 → NH2 + H. I. MRD CI potential energy surfaces (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 429-440 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: To provide a potential energy surface (PES) for a three-dimensional vibrational model of the dissociation dynamics of ammonia in the à state, accurate MRD-CI computations were carried out for a wide range of displacements in the out-of-plane and in-plane bending coordinates along the H(bond)NH2 dissociation coordinate. Some characteristic features of the Ã-state PES, such as the height and position of the barrier to dissociation and the dependence of the PES on the out-of-plane angle, differ noticeably from earlier work on this system. The values for the height of the barrier computed from the MRD-CI extrapolation, full CI, and secular equation estimates are 0.38, 0.34, and 0.32 eV, respectively. © 1996 John Wiley & Sons, Inc.
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  • 118
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    Numerical investigation of a lower bound to the Thomas - Fermi kinetic energy and the validity of the Lieb conjecture for H-, He, Li+, B3+, O6+, Ne8+, and Mg12+ (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 449-452 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The purpose of the present work was twofold: First, a lower bound of Gálvez and Dehesa to the Thomas-Fermi kinetic energy was investigated for the two-electron systems H-, He, Li+, B3+, O6+, Ne8+, and Mg12+. Second, a conjecture of Lieb, relating the kinetic energy to the Thomas-Fermi kinetic energy, was examined. For both investigations, the analytical approximations of Benesch to the radial electron densities of the two-electron systems were used. These approximations are based on the explicitly correlated (Hylleraas-type) 20 variational parameter wave functions of Hart and Herzberg. © 1996 John Wiley & Sons, Inc.
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  • 119
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    CI study of rovibrational dependence of nuclear quadrupole coupling constants of all isotopic variants of OH+ in the X3Σ- state (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 441-448 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 17O and 2H quadrupole coupling constants of rovibrational levels of 17O1H+, 17O2H+, and 16O2H+ in their X3Σ- state have been calculated from molecular wave functions that explicitly describe nuclear motion. The 17O quadrupole coupling is predicted to be strong and its vibrational dependence differs from that known for other nuclei A in the first-row hydrides AH or AH+. The deuterium coupling in 17O2H+ and 16O2H+ is found to be weak and its behavior is similar to that of other first-row hydrides. The change with rotational excitation is unimportant. The quadrupole hyperfine patterns of 17O2H+ in its ground state are dominated by the strong oxygen coupling. © 1996 John Wiley & Sons, Inc.
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  • 120
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    A two-step method of calculation of the electronic structure of molecules with heavy atoms: Theoretical aspect (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 453-463 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An approach for a space-separated calculation of the wave function in the valence and core regions of a molecule is proposed. As the first step, the calculation of the orbitals (or two-component spinors in the relativistic case) in the valence region by the effective core potential (ECP) method is performed. Then, it is followed by a restoration of orbitals (four-component spinors) expanded on spherical harmonics in the core regions of heavy atoms. Theoretical questions of the variational calculation of the molecular orbitals are considered in some core region limited by a sphere. Inclusion from the electronic cloud outside this region is reduced by the necessity of taking into account the orthonormality and boundary conditions together with an effective external field in respect to the selected core region.This method may be used for calculation of matrix elements of operators that are singular near nuclei (P, T-odd interactions, hyperfine structure, etc.). A substantial computational saving can be reached because the method enables, by the most optimal way, to combine the advantages of two well-developed approaches: molecular ECP calculations in the Gaussian basis set and one-center numerical atomic calculations with an external field. It is especially important when the relativistic effects are taken into account. © 1996 John Wiley & Sons, Inc.
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  • 121
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    Relativistic effects on the electronic structure and bonding of [Ir(CN)5]3- (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 471-479 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Four-component relativistic and nonrelativistic molecular orbital calculations were performed for the covalent paramagnetic complex [Ir(CN)5]3-, employing the self-consistent discrete variational method, in the framework of density functional theory. Relativistic effects on the electronic structure and chemical bonding are discussed by comparison of relativistic and nonrelativistic one-electron energy levels, populations, and bond orders. The influence of relativistic effects on calculated absorption energies of the electronic spectrum is briefly assessed. © 1996 John Wiley & Sons, Inc.
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  • 122
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    A semiempirical study of C24N4 and its boron-nitrogen analogs (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 465-470 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of semiempirical calculations on the heterofullerence C24N4 and its boron-nitrogen analogs B6N10C12 (the smallest CBN ball) are presented in accordance with considerations of chemical bondings and geometries. The structures and relative properties of the 28-atom cages, C28, C24N4, and B6N10C12, were investigated using semiempirical methods contained in the MOPAC program. C24N4 and B6N10C12 have Td and C3 symmetry, respectively. B6N10C12 has four geometric isomers, and their optimized structures were determined. The ionization energies, vibrational frequencies, IR intensities, and various thermodynamic properties (including entropy and heat capacity) for C24N4 and B6N10C12 were calculated by the MNDO method. The evidence suggests that a B6N10C12 cage is more stable than that of C24N4. Possible experimental methods to prepare B6N10C12 are also proposed. © 1996 John Wiley & Sons, Inc.
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    A theoretical study on CO2 insertion into an M(bond)H bond (M = Rh and Cu) (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 481-491 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Insertion of CO2 into the transition metal-hydride bond of [RhIIIH2(PH3)3]+, CuIH(PH3)2, and RhIH(PH3)3 was theoretically investigated with ab initio MO/MP4, SD-CI, and CCD methods. The geometries of reactants, transition states (TS), and products were optimized at the Hartree-Fock level, and then MP4, SD-CI, and CCD calculations were performed on those optimized structures. The TS of the CO2 insertion into the CuI(bond)H bond is the most reactantlike, while the TS of the CO2 insertion into the RhIII(bond)H bond is the most productlike. The activation energy (Ea) and the reaction energy (ΔE) were calculated to be 6.5 and -33.5 kcal/mol for the CO2 insertion into the Cu1(bond)H bond, 21.2 and -7.0 kcal/mol for the CO2 insertion into the Rh1(bond)H bond, and 51.3 and -1.1 kcal/mol for the Rh111(bond)H bond at the SD-CI level, where negative ΔE represents exothermicity. These results are discussed in terms of the M(bond)H bond energy and the trans-influence of the hydride ligand. © 1996 John Wiley & Sons, Inc.
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    Ab initio study of the effect of CH ··· O hydrogen bonding on the exo/endo stereoselectivity of Diels-Alder reactions of 2-substituted-1,3-dienes with sulfur dioxide (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 493-499 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations at the MP2/6-31G*//HF/3-21G* level have been carried out to study Diels-Alder reactions of 2-substituted-1,3-dienes with sulfur dioxide. The CH ··· O electrostatic interaction detected in some of the transition structures located could be decisive in the control of the exo/endo stereoselectivity of this type of reaction. © 1996 John Wiley & Sons, Inc.
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996) 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560570301
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    Pair populations and effective valencies from ab initio SCF and spin-coupled wave functions (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 501-518 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A recently proposed pair population scheme is investigated for ab initio SCF and spin-coupled wave functions. All the required expressions are rederived, in very compact form, and efficient computational schemes are presented for their evaluation. As representative examples of different types of bonding, pair populations and related quantities are examined for H2, LiH, BeH, CH4, H2O, and B2H6. Various “effective valencies” are also considered. © 1996 John Wiley & Sons, Inc.
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    Spin-catalysis phenomena (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 519-532 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the grounds of results from quantum chemical calculations, we introduce spin-catalysis as a new concept, and classify different types of chemical reactions according to this concept. We propose a general definition of spin-catalysis as phenomena in which chemical reactions are promoted by substances which assist in overcoming spin-prohibition or in which the activation barrier is lowered through spin uncoupling induced by a paramagnetic catalyst. A number of known phenomena fall into this definition and can be classified according to two main categories: (1) spin-orbit coupling induced and (2) paramagnetic-exchange induced spin-catalysis. Other types of spin-catalysis are also discussed: (3) processes with participation of substances which assist in (a) photochemical and (b) thermal or electrochemical generation of active particles by energy and - by electron transfer [radicals, diradicals, O2(1Δg), etc.], reacting further without spin-prohibition: (4) processes induced by an external magnetic field. Processes (3) and (4) are quite general and well known; their inclusion in the spin-catalysis classification does not introduce any new findings for the chemical kinetics, but the general features, which unite them with the important catalytic processes of the first two types, serve as a useful guide in catalysis theory. Few models of spin-catalytic processes have been simulated for the purpose of illuminating the principles of spin-catalysis; the cis-trans isomerization of ethylene catalyzed by molecular oxygen, the external heavy atom effect in ethylene photochemistry, and some others. © 1996 John Wiley & Sons, Inc.
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    A study of the performance of numerical basis sets in DFT calculations on sulfur-containing molecules (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 533-542 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The performance of numerical basis sets in relation to Gaussian basis sets is examined, by studying 20 small sulfur-containing molecules. The results of geometry optimization calculations are reported for each molecule using both density functional and Hartee-Fock methods. In comparison with experimental data, it is shown that the use of numerical bases tend to overestimate structural parameters, particularly bond lengths, and, in most cases, more than Gaussian basis sets. It is also shown that the use of a larger Gaussian basis set in DFT calculations has the effect of reducing bond lengths. © 1996 John Wiley & Sons, Inc.
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    Static characteristics of the ionization event in the He(23S)-HD collision system (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 543-550 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Vibrational population factors for the nascent Penning ions HD+ (v′)(… He) and energy of the corresponding Penning electrons are calculated for the ionization event He(23S)(SINGLEBOND)HD(v′ = 0) → [He … HD+(v′)] + e- taking place at a range of the He*(SINGLEBOND)HD separations and orientations accessible by the system during thermal energy collisions. The vibrational population factors are obtained from the local widths of the He(23S)(SINGLEBOND)HD(v′ = 0, N) state with respect to autoionization to HD+(… He) in its v′th vibrational level. The initial overall picture of the autoionization event is consistent with the He(23S)(SINGLEBOND)H2(v′ = 0) one. On the other hand, the vibrational population factors are different from the approximate average populations used in initial model theoretical considerations about the Penning processes in the system. Variation of the calculated considerations about the Penning processes in the system. Variation of the calculated quantities with changes in the He*(SINGLEBOND)HD separations and orientations is found to be smooth enough to guarantee that the present data might form a sound basis for construction of analytical representations of the corresponding 2D surfaces and for future study of the dynamics of the collision system. © 1996 John Wiley & Sons, Inc.
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    Theoretically calculated rovibronic transition spectra of KRb (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 575-585 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spectroscopic properties of all the electronic states of KRb dissociating into 4s(K) + 5s(Rb), 4s(K) + 5p(Rb), 4p(K) + 5s(Rb), and 4s(K) + 4d(Rb) and some higher-lying excited states are studied with ab initio calculations. Spectroscopic constants, dipole moments, and the nature of the electronic wave functions for these states are reported. Intensities for the singlet-singlet and triplet-triplet transitions are theoretically calculated from the potential energy curves and the transition dipole moments. © 1996 John Wiley & Sons, Inc.
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    High-level ab initio calculations on CH+2(2A1) + PO(2II) reactions (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 559-566 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present ab initio calculations carried out in the framework of the G2 theory on the singlet and triplet potential energy surfaces corresponding to the gas-phase between CH+2 and PO. The global minimum of both potential energy surfaces is a cyclic singlet-state cation. Oxygen attachment of PO to CH+2 in a triplet configuration is accompanied by a P(SINGLEBOND)O bond fission, with the result that the corresponding global minimum is an ion-dipole complex between P+(3P) and formaldehyde. This is also consistent with the fact that our results predict the formation of formaldehyde to be highly exothermic, either as a neutral or as radical cation. Both charge-transfer processes yielding CH2(3B1) or CH2(1A1) are also exothermic. The formation of other carbon and oxygen containing species are endothermic. © 1996 John Wiley & Sons, Inc.
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    On the thermodynamic characteristics of the benzene … Ar2 complex: An application of the ab initio intermolecular potential (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 551-557 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The benzene … Ar2 cluster was studied by constant energy molecular dynamics simulations using the ab initio intermolecular potential for the benzene … Ar interaction. The relative population of the energetically less favorable one-sided structure was found to be higher that the population of the global minimum (two-sided structure). The preference of the energetically less stable structure was explained by the entropy effect. The thermodynamic functions (entropy, internal energy, and free energy) were calculated independently by integration of the canonical distribution function. © 1996 John Wiley & Sons, Inc.
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    A study of coronene - coronene association using atom - atom pair potentials (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 567-573 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A study of the coronene - coronene association using different interaction potentials based on an atom-atom pair potential proposed by Fraga has been performed. The interaction potentials employed differ in the way the electrostatic and/or dispersion contributions are computed. The influence of both contributions on the geometries predicted for the coronene dimer is discussed in order to analyze the effectiveness of the different interaction potentials. The stationary points found in each interaction energy hypersurface are characterized by calculating the Hessian eigenvalues. Results are discussed in the light of those previously reported for the benzene dimer. Stacked-displaced structures are suggested to be the preferred conformations for the coronene - coronene association. © 1996 John Wiley & Sons, Inc.
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    Theoretical study of spectroscopic properties of bicyclobutane, tricyclopentane, tricyclohexane, and octabisvalene (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 587-594 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The outer-valence ionization potentials, low-lying electronically excited states, and 1J(CC) nuclear spin-spin coupling constants of bicyclobutane, tricyclopentane, tricyclohexane, and octabisvalene have been investigated at the ab initio level by using the outer-valence Green function (OVGF), random-phase approximation (RPA), and equations-of-motions (EOM) approaches, which incorporate the main dynamical correlation effects. A comprehensive theoretical prediction of these spectroscopic observables is presented, which shows several interesting aspects stemming from the unique electronic structure of the bridge bond. © 1996 John Wiley & Sons, Inc.
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    State-selective electron capture processes in collisions involving boron ions (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 611-623 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A full theoretical treatment of electron capture processes using ab initio configuration interaction methods within, according to the collision energy range concerned, a semiclassical or a quantal collisional formalism including translation effects has been developed recently. An application for collisions involving boron, an important impurity in fusion reactors, is presented on examples of the ground state: B3+(1s2) + He, B4+(1s) + H, and the metastable ion B3+(1s2s) + H reactions. © 1996 John Wiley & Sons, Inc.
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    Understanding relativistic effects of chemical bonding (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 641-653 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: To elucidate the physical origin of relativistic changes of molecular properties, exact theorems, perturbation theory, and Hartree-Fock-Slater-Pauli calculations are exploited. The relativistic molecular virial theorem offers insight into the relativistic and nonrelativistic, kinetic, and potential energy contributions to the bond energy. In general, there exist two contributions to the relativistic correction of a molecular property: the relativistic change at the nonrelativistic equilibrium geometry and the change of the nonrelativistic property due to the relativistic change of the equilibrium geometry. Sometimes the first and sometimes the second contribution is the dominant one. Accurate numerical results for H+2-like systems are obtained using direct relativistic double perturbation theory. In some cases, near-degenerate perturbation theory is mandatory. Relativistic changes of chemical bond energies are often proportional to the density change in the K-shell when the bond is formed. Relativistic corrections to many properties (and also to the 1s2-correlation energy) are often proportional to Z2α2. © 1996 John Wiley & Sons, Inc.
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    From laser control of vibrationally mediated photodissociation to photodesorption: Model simulations of breaking metal-ligand bonds in organometallic molecules, clusters, and adsorbates at surfaces (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 595-609 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three specific model systems, HCo(CO)4, Na · NH3, and NO/Pt(111), are used to extend the strategy of vibrationally mediated photodissociations of organometallics, via small clusters of metal atoms and small molecules, to photodesorption of small molecules from metal surfaces. All systems and strategies are similar with respect to breaking metal-ligand bonds by means of infrared IR and visible or ultraviolet UV photons. Specific properties of the systems call, however, for different implementations of the overall tools. In the case of HCo(CO)4, traditional continuous wave (CW) IR + UV 2-photon excitations enhance the rates of HCo bond homolysis. A detailed analysis discovers three effects which result from Franck-Condon transitions in the domains of vibrationally excited wave functions: (i) ultrafast (≈ 20 fs) bond rupture starting from the steeply repulsive wall of the potential energy surface of the excited singlet state; (ii) efficient fast (≈ 200 fs) predissociation via tunneling through neighboring potential barriers; and (iii) decreasing contributions from indirect dissociations via slow (≈ 46 ps) intersystem crossing induced by spin-orbit coupling. In the case of Na · NH3, we suggest a vibrationally mediated pump-and-dump scheme, similar to the strategy of Tannor, Rice, and Kosloff, with proper control of the delay (ca. 70 fs) between ultrashort (ca. 30 fs) pump-and-dump laser pulses. Ultimately, this strategy shifts specific lobes of the vibrationally excited wave packets into a steeply repulsive wall of the potential energy surface of the electronic ground state, with subsequent fast (ca. 100 fs) ruptures of the NA(SINGLEBOND)NH3 bond, similar to effect (i) for HCo(CO)4. Finally, we show that a similar, vibrationally mediated pump-and-dump scheme may also support photodesorption of NO from Pt(111), with an intrinsic relaxation step for the electronically excited system NO/Pt(111) instead of active pump-and-dump control for Na · NH3. All strategies are simulated by fast Fourier transform propagations of representative wave packets on two potential energy surfaces. © 1996 John Wiley & Sons, Inc.
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    A classification of open-shell states of molecules (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 625-640 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A proposal for the classification of molecular open-shell states is presented. This classification can be applied to most standard situations, well described within the Born-Oppenheimer and nonrelativistic approximations. The main idea is to consider electronic configurations important in the CI-type expansion of the state wave function at the equilibrium conformation, as well as along any generalized coordinate describing a physically realistic conformational transformation, in particular, the system separation in the state considered. The states are generally divided into single- and multireference ones. On the basis of the conformational dependence of the multireference character, the states are further divided into locally, asympotically, globally, and strongly multiconfigurational ones. The proposed classification is an attempt at a unifying view of such well-known phenomena, like avoided crossings, Woodward-Hoffman forbidden processes, conical intersections, Jahn-Teller stabilizations, and van der Waals minima. A comparison with the classification of Kutzelnigg and Smith [Int. J. Quantum Chem. 2, 531 (1968)], based on an analysis of natural orbital occupation numbers, is made wherever possible. © 1996 John Wiley & Sons, Inc.
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    Static and dynamic density functional investigation of hydrated beryllium dications (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 655-662 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using density functional theory with Becke's gradient correction for the exchange part and Vanderbilt's ultrasoft pseudopotentials, we investigated [Be(H2O)n]2+ clusters for n = 1 and 3. A new scheme implemented with the Car-Parrinello method is used, upon which no periodic boundary conditions are imposed so that isolated and possibly charged molecules can also be treated dynamically in a plane-wave basis. Harmonic vibrational frequencies are obtained via a fit of the molecular dynamics trajectory in terms of harmonic oscillators, for which we use a fragmentation scheme to analyze complex spectra. Based on this combination of techniques, we find good agreement with data from SCF calculations and correlated methods. © 1996 John Wiley & Sons, Inc.
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    Crystal-field splittings and optical spectra of transition-metal mixed-ligand complexes by effective Hamiltonian method (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 663-671 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Many of the important properties of transition-metal complexes depend on the low-energy excitation spectrum formed by d-electrons of the central transition-metal atom. The spectra of this type are usually fit to the well-known crystal field theory or to the angular overlap model. The result of the fitting is a set of parameters which are considered as characteristics of the electronic structure of the complex such as strength of the ligand field or types and extent of metal-ligand bonding. We present here a short account of the effective Hamiltonian method recently developed to calculate the splitting of the d-levels by the ligands and the resulting d-d spectra of transition-metal complexes together with some results of its application to the mixed-ligand complexes with the general formula ML4Z2, where M = V, Co, Ni; L = H2O, NH3, Py; and Z = H2O, NCS-,C -l. Particular attention is paid to the V(H2O)4Cl2 and Co(H2O)4Cl2 compounds. The former seems to have tetragonal structure, whereas for the latter, our method predicts a spatially degenerate ground state for the tetragonal arrangement of the ligands. That must lead to the Jahn-Teller distortion, which is actually observed. © 1996 John Wiley & Sons, Inc.
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    Density functional theory of chemical reactivity indices in some ion - molecule reaction systems (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 673-682 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three isoelectronic reactions, proton transfer (PT), hydrogen abstraction (HA), and electron transfer (ET), of NH+3 with NH,3 H2O, and HF have been studied using ab initio molecular orbital calculations. For the reaction of NH+3 + H2O, the energy of the transition state (TS) is higher than that of the reactants. This is consistent with the experimental observation that the rate constant is less than the average dipole orientation (ADO) rate constant. It seems reasonable that the reaction rate for the reaction NH+3 + H2O would hardly depend on the v2 mode of NH+3 at least for low-lying excited states (Eint≤ 0.714 eV) of the v2 mode, because the v2 mode contributes mainly to the normal mode orthogonal to the reaction coordinate at the TS. This is consistent with experimental observation. A similar prediction can be made for the NH+3 + HF reaction. The electron-transfer processes for the HA reactions have been examined in terms of the intrinsic reaction coordinate (IRC). The order of reactivity with NH+3 is NH3 〉 H2O 〉 HF. It is found that the degree of the electron transfer and the reactivity are correlated with the absolute hardness (η) of NH3, H2O, and HF. This is in accord with the softness as the chemical reactivity index in the density functional theory. © 1996 John Wiley & Sons, Inc.
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    Relativistic many-body and QED calculations on atomic systems (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 683-695 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A review is given of many-body perturbation methods, particularly in the all-order and coupled-cluster forms. Relativistic many-body schemes are analyzed in terms of one- and two-photon potentials, derived by means of QED. A complete second-order (nonradiative) calculation for He-like ions is presented, including repeated Breit interactions as well as the effects of retardation and of negative-energy states, but omitting the Lamb shift. Numerical results of some Lamb-shift calculations are also given. From the analysis, conclusions can be drawn concerning the accuracy of certain relativistic many-body approaches. © 1996 John Wiley & Sons, Inc.
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    Theoretical studies of structure, stability, and chemical bonding in oxohydride OM4H6 complexes (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 697-705 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of ab initio RHF/3-21G, RHF/6-31G*, and MP2/6-31G** / / HF/6-31G* calculations for 10 possible configurations of OM4H6 molecules (MO · 3MH2, M = Be, Mg) are reported. Five isomers of OBe4H6 and three isomers of OMg4H6 have been found within an energy range of ã 15 kcal mol-1. The “lanternlike” C3v structure is the most favorable one for both complexes. Both molecules OM4H6 are stable to decomposition through all of the studied pathways. Chemical bonding in the OMk polyhedra containing two-, three-and four-coordinated oxygen atoms is discussed. © 1996 John Wiley & Sons, Inc.
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    Energies and structures of rotating argon clusters: Analytic descriptions and numerical simulations (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 707-714 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The dependence of the rotational energy of small argon clusters on the magnitude and direction of their rotational angular momenta is obtained by two different methods, namely, by analytic descriptions parametric in structural variables (centrifugal displacements) and by classical simulations carried out in rotating frames so that rotational angular momenta are conserved. Potential energies are taken as additive Ar2 pair potentials [R. A. Aziz, J. Chem. Phys. 99, 4518 (1993)], augmented in some cases by three-body Axilrod-Teller interactions, thus complementing our earlier studies of rare-gas clusters modeled by additive Lennard-Jones oscillator (LJO) pair potentials [L. L. Lohr and C. H. Huben, J. Chem. Phys. 99, 6369 (1993)]. Quartic and sextic spectroscopic constants are found to be approximately 10% smaller when the Aziz pair potential is used, reflecting its greater stiffness as compared to the LJO potential. The sign of the sextic tensor coefficient for both tetrahedral Ar4 and octahedral Ar6 is such that for sufficiently high J the C2v (or D2h) structures with J parallel to a pseudo-C2 (or true C2) axis (saddle points on the rotational energy surface at low J) become local energy maxima, the D2d (or D4h) structures with J parallel to an S4 (or C4) axis representing the energy minima. The trigonal bipyramidal cluster Ar5 resembles both Ar3 and Ar4 in its rotational characteristics but with reduced manifestations of nonrigidity. As found with an LJO pair potential [D. H. Li and J. Jellinek, Z. Phys. D 12, 177 (1989)], the icosahedral Ar13 cluster displays a very slight preference for D3d structures with J parallel to a C3 axis, while the D5d structures with J parallel to a C5 axis are energy maxima and the D2h structures with J parallel to a C2 axis are saddle points on the rotational energy surface. The scalar quartic spectroscopic coefficient for Ar13 is found to be 2.15 × 10-14 times that for the reference diatomic Ar2. A variety of structural instabilities are described for Ar13 clusters with very high rotational energies. © 1996 John Wiley & Sons, Inc.
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    Theoretical studies on the elimination of hydrogen fluoride from alkyl fluoride and its substituent effect (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 715-719 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction mechanism of the α, α and α, β elimination of hydrogen fluorides from alkyl fluorides has been studied theoretically. For fluoroethane as a reactant, the transition state (TS) optimized at the level of the 6-31G** basis set shows that the α, β elimination proceeds via a four membered-ring TS with a barrier height 64.6 kcal/mol, while the α, α elimination, via a three-membered ring TS with a 83.7 kcal/mol barrier. Four substituents, CH3, CN, F, and NH2, were used to investigate the substituent effect of elimination by using the 3-21G basis set. The calculated barriers show that NH2-substituted alkyl fluorides favor both the α, α and α, β elimination and these two reactions would be expected to proceed simultaneously. © 1996 John Wiley & Sons, Inc.
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    Platinum-centered octakis (triphenylphosphino gold) clusters: A relativistic MO study (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 735-740 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The system of [Pt(CO)(AuPPh3)8]2+ represents an example of the spheroidal 18-electron cluster which consists of 283 atoms, 817 real-valence orbitals, and, eventually, 1634 complex spin-orbitals. The above system was investigated by an MO-LCAO-SCF study at the all-valence (nonempirical) quasi-relativistic and relativistic CNDO/1 levels. Owing to the complexity of the system under study, instead of an orbital interaction diagram, the density-of-state functions were generated. Their projections, obtained through net orbital populations, bear information about the positioning of MOS and eventual interaction of the AOS composing individual MOS. The platinum 5d orbitals exhibit a mixing with those of gold and contribute to only a few HOMOS; the rest of gold 5d orbitals, however, are situated at lower energies. © 1996 John Wiley & Sons, Inc.
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    Ab initio study of lithiated N-methylpyridones (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 729-733 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations were performed on the N-methylpyridones lithiated on the aromatic ring using a 6-31G* basis set. Whenever the lithium atom is on a carbon adjacent to the carbonylic group, a bridged structure is obtained where lithium is coordinated to both carbon and oxygen; these structures are the most stable isomers. © 1996 John Wiley & Sons, Inc.
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    Tautomerism in 2,2′-bipyridyl-3,3′-diol (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 721-728 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three stable tautomeric forms, dienol (DE), ketoenol (KE), and diketo (DK), of 2,2′-bipyridyl-3,3′-diol BP(OH)2 were found in this study, using the semiempirical AM1 and MNDO-PM3 and ab initio (4-31G basis set) methods. All calculations were carried out without any symmetry restrictions. There is a good agreement between the ab initio calculated and experimentally obtained structural parameters for the DE tautomer. Transition structures, corresponding to the DK → KE and KE → DE processes have also been found. On the basis of the results from the present work, an asynchronous (two-step) DK → KE → DE mechanism of the IPT reaction in BP(OH)2 is proposed. © 1996 John Wiley & Sons, Inc.
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    An ab initio study on HXC(double bond)O … HY molecular complexes (X, Y = F, Cl) (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 757-766 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Post-Hartree-Fock calculations were carried out to predict the stabilities and properties of four HClCO … HCl, HClCO … HF, HFCO … HCl, and HFCO … HF molecular complexes. Full geometry optimizations and vibrational frequency calculations were performed for all systems using standard 6-311G(d, p) and 6-311G(2d,2p) basis sets at the MP2 level of theory. Single-point calculations of the interaction energies were carried out for all complexes at the MP4(SDTQ) level with the 6-311G(d, p) basis set. All systems were found to be stable and their predicted molecular parameters match well available (very scarce) experimental data. © 1996 John Wiley & Sons, Inc.
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    Computer modeling of assembly of atoms in an electric field (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 741-755 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A computer modeling of events that occurred under the scanning tunneling microscope (STM) tip was considered from a chemical standpoint. The DYQUAMOD, semiempirical dynamical-quantum chemical program system, which well manifested itself in a quantitative studying of nano-sized objects, was transformed into the DYQUAFIELD program system involving an external electrostatic field. The fields of changeable configurations were simulated by a set of up to 200 point charges. Atomic system modeling was focused on obtaining local electron density (LED) as well as on the object chemical transformations caused by the field. This article presents the results of simulating LED of differently originated graphite films, of a field action on a set of stable molecules, and of an irreversible in-field reconstruction of the diamondlike film surface, which demonstrates a possible nano-sized memory element creation. © 1996 John Wiley & Sons, Inc.
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    Quantum chemical study of self-organized water fractals (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 767-773 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The energetically possible ways of water condensation at the molecular level were considered. The quantum chemical calculations of the relative stability of elementary modules of the fractal structure without external fields using AM1, CNDO/2, and MNDO methods were done. It was shown that CNDO/2 gives the most adequate results for simple water clusters. Condensation energy per molecule was chosen as a criterion of the cluster stability. It was shown that the twisted modules are the stable structures with respect to the water dissociation. In comparing the energies of the icelike and twisted structures, we conclude that the latter may exist under certain conditions. © 1996 John Wiley & Sons, Inc.
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    Application of the localized representation for studying interaction energies (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 775-780 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It has already been shown that the use of the localized many-body perturbation theory (LMBPT) makes its possible to calculate the interaction energy at the correlated level in a straightfoward way. In this article, we show that the correlated part of the interaction energy, furthermore, can simply be decomposed into dispersion and charge-transfer contributions using the LMBPT scheme. The CH2O + NH3 model system was chosen for the above study. © 1996 John Wiley & Sons, Inc.
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    Molecular orbital studies on the spin states of nitroxide species: Bis- and trisnitroxymetaphenylene, 1,1-bisnitroxyphenylethylene, and 4,6-dimethoxy-1,3-dialkylnitroxy-benzenes (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 781-799 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We describe by approximate MO calculation a number of species for which high spin states are either the ground state or lie very low in energy. These are models for the spin coupling in projected ferromagnetic organic materials. The theory guiding their construction is based on planar conjugated systems, while experimentally realized systems are often far from planarity. We can by appeal to steric decoupling explain the failure of the prediction that methoxy subsitution on metaphenylene-coupled nitroxides should stabilize the triplet. However, we find in general that the qualitative rules derived from discussion of planar systems are robust; drastic departures from planarity are required before they lose their value. © 1996 John Wiley & Sons, Inc.
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996) 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560570401
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    STOP: A slater-type orbital package for molecular electronic structure determination (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 801-810 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general ab initio package using Slater-type atomic orbitals is presented. This package, called STOP, uses the one-center two-range expansion method to evaluate the multicenter electronic integrals. Thoroughly optimized numerical techniques, in particular, convergence accelerators and suitable Gauss quadratures, are used in the algorithms which provide accurate numerical values for all these integrals. STOP thus provides wavefunctions for general molecular structures at the self-consistent field level for the first time over a Slater-type orbital basis. © 1996 John Wiley & Sons, Inc.
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    Hydrogen interaction with shallow and deep centers in GaAs (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 823-841 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of detailed theoretical investigations of the properties of atomic and diatomic H in GaAs were analyzed with the effort to give a unified picture of the H behavior in this semiconductor. All calculations were performed in the pseudopotential density-functional framework using a supercell approach. We studied both shallow impurities (Si and C) and deep point defects (As antisite and Ga vacancy). Generally, a simple scheme may be applied in order to describe the H interaction with shallow impurities, where a key role is played by the amphoteric character of H. More complex mechanisms are involved in the deep impurity case that are related to new, interesting effects of H incorporation in GaAs. © 1996 John Wiley & Sons, Inc.
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    Ab initio dynamic polarizabilities of polymers. I. Hydrogen chain models (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 811-821 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio dynamic polarizabilities per unit cell of infinite stereoregular molecular hydrogen chains are calculated at the coupled Hartree-Fock level of approximation by using the random-phase approximation and the STO-3G and double-zeta atomic basis sets. Comparison with molecular calculations on increasingly large oligomeric chains emphasizes the nice extrapolation property of the polymeric technique that provides asymptotic values very close to the largest oligomeric values. The poles of the polarization propagator associated with the electric dipole polarizability correspond to the singlet excitation energies. Comparisons are performed with other techniques that provide the band gap. © 1996 John Wiley & Sons, Inc.
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    Ab initio crystal orbital calculations on three-dimensional crystals of large bioorganic molecules and polymers (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 851-860 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio crystal orbital calculations on three-dimensional crystals of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), diprotonated deoxycytidine-5′-monophosphate monohydrate (5′-dCMP*2H*1Water), disodium deoxyguanosine-5′-monophosphate tetrahydrate (5′-dGMP*2Na*4Water), disodium uridine-3′-monophosphate tetrahydrate (3′-UMP*2Na*4Water), monosodium monoprotonated deoxyadenosine-5′-monophosphate hexahydrate (5′-dAMP*1H*1Na*6Water), disodium deoxycytidine-5′-monophosphate heptahydrate (5′-dCMP*2Na*7Water), cis-polyacetylene (cis-PA), and polythiophene (PTP) were carried out using the CRYSTAL92 routine package. A suitable basis set has been found that enables one to carry out the above calculations at the STO-3G level of accuracy using SILICON GRAPHICS workstations. However, to obtain reasonable results for three-dimensional crystals of polymers, one has to use much more extended basis sets. © 1996 John Wiley & Sons, Inc.
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    Modeling of interaction properties of surfaces of phyllosilicates: A theoretical forecast of adsorption isotherms of noble gases at the talc surface (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 843-849 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electrostatic potential and the intensity of the electric field above the isolated layer of the phyllosilicate mineral talc (layer group symmetry C2/m) were computed using the semiempirical INDO/2 method. The electrostatic potential Vc and the intensity of electric field (OVERBAR)Ec, above the surface of semi-infinite crystal were obtained as the sum of the contributions of the infinite number of individual layers. The interaction energy Uint between a noble gas atom and the talc crystal was computed as (a) a pure Lennard-Jones energy ULJ and (b) the sum of ULJ and interaction energy with induced dipole moment: Uind = α|(OVERBAR)E|2, where α is the dipole polarizability of the noble gas atom. The one-particle configurational integral was calculated within the classical mechanics limit. Both the free and adsorbed gas phases were assumed to be ideal. The obtained results demonstrate that in the case of uncharged and nonpolar adsorbate and an uncharged surface the electrostatic part of the interaction energy should be included in the total interaction energy. © 1996 John Wiley & Sons, Inc.
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    RHF Ab initio electronic and molecular structures of a ((SINGLEBOND)Be2(SINGLEBOND))∞ chain (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 871-879 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio RHF/STO-3G, 3-21G, and 6-31G band structure calculations were carried out on an extended regular chain of beryllium atoms ((SINGLEBOND)Be2(SINGLEBOND))∞ to study its stability in comparison with corresponding data on finite chains and previously reported results on small elemental beryllium clusters. It is found that, starting from a linear distribution of atoms along the chain, the system evolves toward a regular zigzag planar structure. The total RHF energy per constituent atom is higher for the chains than for the clusters and is thereby suggestive of that isolated chains are unlikely to exist except in constraining channels of host structures. In the limit of the infinite chain, there are signs indicating that the RHF model is no longer an adequate description. © 1996 John Wiley & Sons, Inc.
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    Hydrogen chemisorption on Pt/Ni (111) systems (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 887-896 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Extensive self-consistent real-space recursion-method calculations were performed for the Pt overlayer or the Pt7 cluster on the Ni(111) surface and for hydrogen chemisorption on these systems. Correlations between the surface-atom local density of electronic-state properties before chemisorption, surface reactivity, and the initial-state contribution to the metal core-level shifts, respectively, is documented and discussed. The experimentally observed catalytic properties of Pt—Ni alloy surfaces are also briefly considered. © 1996 John Wiley & Sons, Inc.
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    H2O and H2 interaction with ZnO surfaces: A MNDO, AM1, and PM3 theoretical study with large cluster models (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 861-870 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We investigated the adsorption and heterolytic dissociation of H2O and H2 molecules on a (ZnO)22 cluster corresponding to ZnO (0001), (000(OVERBAR)1), and (10(OVERBAR)10) surfaces using MNDO, AM1 and PM3 semiempirical procedures. The geometry of the adsorbed molecule has been optimized in order to analyze binding energies, charge transfer, and preferential sites of interaction. The adsorbed species interact most strongly when it is bonded to the twofold coordinated zinc atom of the cluster surface. The interaction of the H2O molecule with the surface of ZnO has a charge transfer from H2O to the surface ranging between 0.17 and 0.27 au. The neighboring atoms of the surface are the main receptors during the process of charge transfer. Our results indicate that there is a weak bonding of the hydrogen atom from OH with the oxygen surface atom that could produce the O(SINGLE BOND)H·O band. The interaction of the H2 molecule with the surface is generally weak and only the PM3 method yields a strong binding energy for this interaction. There is a charge transfer from the H2 molecule to the surface. The chemisorption of H on oxygen atom of the surface transfer charge from the surface to the H. We also calculated the vibrational analyses for these interactions on ZnO surface and compared our results with available experimental data. © 1996 John Wiley & Sons, Inc.
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    Phase diagram of atom-vacancy solid solution (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 881-886 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The assumption of the concentration dependence of the energy of formation of vacancy-type defects (VTD) in solids is applied to the procedure of the estimation of the defect interaction parameters. The VTD effective interaction is described by the mixing potential. The sign of this potential characterizes the stability of the VTD-atom “alloy.” Employing the example of Nb, we illustrate the ability of the suggested approach to describe the stability of the VTD-atom “alloy.” © 1996 John Wiley & Sons, Inc.
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    First π-electron transitions in finite polymethine chains (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 913-918 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the present article, polymethine and annulene electronic spectra are described in a unique way by means of the Pariser-Parr-Pople method. The analytical expressions were derived for the first π-electron transitions energies. The character of the annulene spectra, like the character of the open-chain spectra, was shown to be dependent on the ratio between electron and site numbers. The Dahne's triad theory and the existence of finite energy gap in polymethines are discussed. © 1996 John Wiley & Sons, Inc.
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    Effective mixing potentials and ordering thermodynamics of alloys (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 897-901 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the basis of a nonempirical pair potential, the temperature dependencies of the long-range-order parameter of NiTi and AuPd are calculated. The suggested effective potential approach allows one to describe the changes in phase relations. The evaluation of the internal mixing energy of alloys is carried out in the framework of the static concentration waves theory of Khachaturyan. The model outlines the technique to calculate the values of the energy parameters of the ordering process from first principles. © 1996 John Wiley & Sons, Inc.
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    A microscopic study of the quantum critical behavior of the spin-1/2 anisotropic Heisenberg models (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 919-927 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We extend our own previous applications of the microscopic coupled-cluster method (CCM) to quantum antiferromagnets. In particular, we carry out a systematic calculation involving high-order multispin correlations for the spin-1/2 anisotropic Heisenberg models on the one-dimensional chain and the two-dimensional square lattice. Their ground-state properties are obtained as functions of the anisotropy parameter. Our CCM analysis not only produces accurate results for such physical quantities as the ground-state energy which are comparable to the best results from other techniques, but it also enables us to study the quantum phase transitions of the spin models in a systematic and unbiased manner. © 1996 John Wiley & Sons, Inc.
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    Lattice relaxation and order in the low-spin to high-spin transitions in molecular crystals (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 903-912 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of the d6 iron(II) complexes with bulky organic ligands (like [Fe(bipy)2(NCS)2]) can exist in two spin forms: in the low-spin (S = 0) form at low temperature and in the high-spin (S = 2) form at high temperature. In the crystal phase, the transition between these two forms may be either smooth or abrupt. Recently, the abrupt spin transitions were identified with the first-order transitions between different ordered phases occurring in the binary mixtures of the two spin forms of the complex. Here, we apply the method widely used in the field of binary metal alloys to the analysis of the spin transitions. The molecules undergoing the spin transition are modeled by octahedra of variable size which interact when they are immediate neighbors in the crystal lattice. We show that some simple assumptions concerning the intermolecular interaction and crystal geometry relaxation allows one to get the desired first-order phase transitions together with a satisfactory description for the crystal compressibility as a function of temperature. © 1996 John Wiley & Sons, Inc.
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    Ab initio SCF structure investigation of β-hydroxypropionic acid and 3-aminopropionamide (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 929-941 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The potential energy surfaces of β-hydroxypropionic acid and 3-aminopropionamide have been investigated by means of RHF/4-31G calculations. Structures, reaction paths for internal rotations, and the respective energy barriers are reported. The influence of the various intramolecular interactions on structural and energetical properties is shown and compared to the results previously obtained for β-alanine. © 1996 John Wiley & Sons, Inc.
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    An ab initio MO study on the thymine dimer and its radical cation (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 949-957 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio SCF calculations with the 6-31G basis set for the thymine dimer (cys-syn form) and the thymine dimer radical cation are reported. The fusion of the thymine bases at the C5 and C6 positions involves the formation of a cyclobutane ring with puckering. The puckering causes a notable difference in the electronic structures of the two bases of the thymine dimer. The density of the HOMO orbital of the thymine dimer is localized on the O2, N1, and C6 atoms of both thymine rings, with the higher density on one of the rings. The HOMO orbital has a bonding character on the C6(SINGLEBOND)C6 bond. In the thymine dimer radical cation, the unpaired electron is localized mainly on the lengthened C6(SINGLEBOND)C6 bond with the higher density on one of the C6 atoms and to a lesser extent on the N1 atoms of both rings. © 1996 John Wiley & Sons, Inc.
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    DNA base amino groups and their role in molecular interactions: Ab initio and preliminary density functional theory calculations (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 959-970 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Interactions of DNA bases frequently involve the DNA base amino groups. In contrast to the empirical force fields, the ab initio calculations predict nonplanar DNA base amino groups. The same conclusion also follows from the density functional theory (DFT) calculations. Both local and nonlocal density approximations were used. Optimized geometries of two other molecules with nonplanar amino groups (aniline, formamidine) are presented for comparison. The influence of nonplanar DNA base amino groups on the conformational variability of DNA is discussed. © 1996 John Wiley & Sons, Inc.
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    Theoretical studies on the decarboxylation reaction in thiamin catalysis (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 943-948 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformational behavior and the stability of thiazolium and thiamin diphosphate (ThDP) adducts formed by the C2 addition of the substrates pyruvate and glyoxylate to the corresponding thiamin systems are investigated within the force-field version PIMM90 as well as the semiempirical AM1 and PM3 methods. Moreover, the reaction coordinate of the decarboxylation process of the adducts with respect to the C2α(SINGLEBOND)COO- bond are calculated by PM3 and AM1. The calculations on the key intermediates of the Breslow mechanism are performed in order to study the steric aspects of both substrate adducts that show a different pathway in the catalytic cycle. The alternative structural findings for the decarboxylation products are compared with first 6-31C* studies on the corresponding thiazolium model systems. Especially, the PM3 calculations show that the elimination of CO2 is favored if the arrangement of the carboxylate group is nearly perpendicular to the plane of the thiazolium ring. These results support the least-motion maximum-overlap mechanism in the enzymatic decarboxylation reaction, proposed by Kluger. The most stable conformers of the ThDP adducts and its decarboxylation products are characterized by V-like structures and the formation of a significant intramolecular hydrogen bonding under participation of the 4′-aminopyrimidine ring. © 1996 John Wiley & Sons, Inc.
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    The molecular structure of N-hydroxyurea (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 971-974 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations were performed on the tautomers and conformers of N-hydroxyurea using a 6-31G** basis set. The minimum-energy structures have been found and the importance of the intramolecular hydrogen bond as the stabilizing factor was pointed out. © 1996 John Wiley & Sons, Inc.
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996) 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560570501
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    List of participants (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 985-1018 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560570502
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  • 175
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    Approach to nonadiabatic transitions by density matrix evolution and molecular dynamics simulations (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 975-983 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Many biological processes are characterized by an essentially quantum dynamical event, such as electron or proton transfer, in a complex classical environment. To treat such processes properly by computer simulation, allowing nonadiabatic transitions involving excited states, we recently developed a density matrix evolution (DME) method [H. J. C. Berendsen and J. Mavri, J. Phys. Chem, 97, 13464 (1993)] which simulates the dynamics of quantum systems embedded in a classical environment. The formalism of the method is presented and an overview of the applications ranging from collisions of a quantum harmonic oscillator with noble gas atoms to proton tunneling in a double-well hydrogen bond is given. The methodology for treatment of proton-transfer processes with inclusion of excited states is presented. Future application of the method on biologically interesting processes, such as proton transfer in enzymatic reactions, is discussed. © 1996 John Wiley & Sons, Inc.
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    Interpolant polynomial technique applied to the PPP model. I. Asymptotics for excited states of cyclic polyenes in the finite cyclic Hubbard model (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1019-1031 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For theoretical reasons, and on account of the development of a new interpolation technique, it is useful and important to examine the asymptotic behavior of the solution to the one-dimensional Hubbard model. In this article, it is shown how perturbative expansions for the energy can be developed in the asymptotic region of the relevant coupling for all the relevant excited states of cyclic polyene rings. © 1996 John Wiley & Sons, Inc.
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    A BSSE-free SCF algorithm for intermolecular interactions. III. Generalization for three-body systems and for using bond functions (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1049-1055 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The simple BSSE-free SCF method (CHA/F) introduced in the previous parts of this series is extended to the case of three subsystems, which may be either three weakly interacting molecules or a bimolecular system described by using bond functions. The CHA/F formalism is formulated in a more transparent manner, indicating also a straightforward way for generalization to the case of an arbitrary number of subsystems. The illustrative calculations show the viability of using the CHA/F scheme for three-component systems. © 1996 John Wiley & Sons, Inc.
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    Interpolant polynomial technique applied to the PPP model. II. Testing the interpolant technique on the Hubbard model (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1033-1048 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: One is often led, in quantum mechanics, to a perturbative solution of an eigenvalue problem, which is defined by a given Hamiltonian. The perturbative series for the energy which results will be a function of a coupling constant which appears in the Hamiltonian. In this article, the perturbative series for the energy of a state of a cyclic polyene ring which are valid for the small and large coupling limit of the model are used to construct algebraic functions. These algebraic functions are defined in terms of polynomials which are given as a function of the energy variable and coupling parameter and can be solved to give the energy as a function of coupling. It is found that relatively small polynomials give very good agreement with the exact values and that the accuracy of the results increases rapidly as the degree of the polynomial increases. The final goal of this and subsequent articles is to study energy levels in PPP models of planar conjugated hydrocarbons. In this article, we test an interpolant technique on the case of the one-dimensional Hubbard model, where an exact solution can be obtained by solving a system of nonlinear equations. In the case of the Hubbard model, the correlation effects are overestimated. Therefore, if the technique works for the Hubbard model, it is reasonable to assume that the technique would work even better for the PPP model. © 1996 John Wiley & Sons, Inc.
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    Spin-free quantum chemistry. XXVI. The Ising, small-bipolaron theory of cuprate superconductivity (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1057-1066 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Ising, small-bipolaron (ISB) theory is a strong-coupling theory of cuprate superconductivity which is based on the negative-U, Hubbard Hamiltonian. Its ground state is composed of (small) bipolarons and (small-bipolaron) holes with a vibronically induced, bipolaron-hole exchange interaction, JBH, between them. The energy gap, Δ(0), is taken to be equal to the dissociation energy of a small bipolaron and which, since it is defined spectroscopically, is not an order parameter. The application of the Ising mean-field theory to the highly degenerate ground-state yields a second-order phase change with kTC/2 = JBH and a real order parameter, Ω(T), which is valid over the entire temperature range from zero to TC. Near TC, the Ising free-energy functional takes the same form as does the Landau. In the presence of an electromagnetic field, the Ising functional is a generalization of the Ginzburg-Landau functional which employs a complex order parameter and which is invariant under the electromagnetic gauge transformation. The breaking of the gauge invariance yields the London theory of superconductivity. © 1996 John Wiley & Sons, Inc.
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    Solvatochromism of the π* ← n transition of acetone by combined quantum mechanical - classical mechanical calculations (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1067-1076 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The solvent shift of the π* ← n transition of acetone in water, acetonitrile, and tetrachloromethane was calculated in a combined quantum mechanical - classical mechanical approach, using both dielectric continuum and explicit, polarizable molecular solvent models. The explicit modeling of solvent polarizability allows for a separate analysis of electrostatic, induction, and dispersion contributions to the shifts. The calculations confirm the qualitative theories about the mechanisms behind the blue shift in polar solvents and the red shift in nonpolar solvents, the solvation of the ground state due to electrostatic interactions being preferential in the former, and favorable dispersion interaction with the excited state, in the latter case. Good quantitative agreement for the solvent shift between experiment (+1,700, +400, and -350 cm-1 in water, acetonitrile, and tetrachloromethane, respectively) and the explicit solvent model (+1,821, +922, and -381 cm-1) was reached through a modest Monte Carlo sampling of the solvent degrees of freedom. A consistent treatment of the solvent could only be realized in the molecular solvent model. The dielectric-only model needs reparameterization for each solvent. © 1996 John Wiley & Sons, Inc.
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    Model potentials in studies of atomic electron density distribution (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1097-1106 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In density functional theory (DFT), a many-electron problem for the electron density in atoms may be reduced, according to the Kohn-Sham scheme, to a one-electron problem. In the present work, a variational model is proposed which leads, within some assumptions, to the set of equations describing the change of the electron density ρ and energy ε during the ionization process. It is shown that the one-electron density contributions are not necessarily spherically symmetric, but assume the symmetry which depends upon the symmetry of the positive field. A few nonspherically symmetric potentials are studied in the present article. The nonlinear differential equation for density r is formulated and solved for Coulombic, Fues-Kratzer, and Hartmann potentials. The solutions and some particular examples are presented. © 1996 John Wiley & Sons, Inc.
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    Ab initio studies on the mechanism of the fluoroketene-imine cycloaddition reaction: Via a gauche or trans intermediate (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1107-1114 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cycloaddition reaction of FCH(double bond)C(double bond)O and NH2CH(double bond)NH leading to 2-azetidinone was studied theoretically at the level of RHF/6-31G and RHF/6-31G*. Two possible mechanisms via a gauche or trans intermediate were compared. The obtained results show that the reaction proceeds in a gauche manner much easier than in a trans one. © 1996 John Wiley & Sons, Inc.
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    Asymptotic behavior of electron densities and computation of one-electron properties (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1077-1096 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The role of the asymptotic behavior of approximating sequences of electron densities ρn(r) in the calculation of one-electron properties is studied. Rigorous mathematical results in the frame of Hilbert spaces are used to prove the following facts: (i) Both the L2 convergence of wave functions ψn and the E convergence of the corresponding energies En guarantee the correctness of the limiting procedure limn→x ∫Ω s((overline)x(/overline)|ψn|2 d(overline)x(/overline) = ∫Ω s((overline)x(/overline))|ψ|2 d(overline)x(/overline) for the most frequently used operators s(x), Ω being any bounded region of the n-particle configuration space R3N; and (ii) the uniform boundedness of the sequence {ρn} together with both the L2 and E convergencies is sufficient to guarantee the correctness of the limiting procedure limn→x ∫∞0 s(r)ρnr2dr = ∫x0 s(r)ρr2 dr for most one-electron operators s(r) including the power moment operators rk which, for large k, are representative of the class of operators not relatively form-bounded by the Hamiltonian. The mathematical concept of uniform boundedness is used to give a characterization of the capability of {ρn} to reproduce the asymptotic behavior of the true electron density ρ and it is shown by means of numerical examples how a sequence {ρn} that does not reproduce the correct asymptotic behavior is not uniformly bounded and can give divergent expectation values of one-electron operators s(r) not relatively form-bounded by the Hamiltonian. © 1996 John Wiley & Sons, Inc.
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    Cluster models of ionic crystals: Band gaps (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 1115-1119 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We performed electronic ab initio calculations of ion clusters simulating a piece of ionic crystal. Our main interest here is to estimate the (direct) energy band gap from the cluster energy levels. The model consists of a central cation surrounded by four shells of ions, with additional point charges embedding the cluster. We present results for the series of alkali halides with rock-salt structure. © 1996 John Wiley & Sons, Inc.
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996) 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560570601
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    Structural and mechanistic organic chemistry: A tribute to professor Norman L. Allinger (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 715-715 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560600205
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  • 187
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    New “localized orbitals” appropriate for post-Hartree - Fock calculations (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 731-742 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this work, we propose a new orbital set based on density matrices of subsystems. Most of the resultant orbitals are localized in the subsystems and resemble the natural orbitals of the subsystems. Correlation between two electrons, each in a distant localized orbital, should be small and the corresponding configurations can be neglected in a calculation. Test calculations are performed on hexatriene and the usefulness of these orbitals is shown. © 1996 John Wiley & Sons, Inc.
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996) 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560600301
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    An alternative way of solving secular equations for the Hamiltonian matrices of regular quasi-one-dimensional systems (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 717-729 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An alternative approach to secular problems for Hamiltonian matrices H of regular quasi-one-dimensional systems is suggested. The essence of this approach consists of the inverted order of operations against that of the traditional solid-state theory, viz., taking into account the local structure of the system is followed by regarding the translational symmetry of the whole chain. The first step is performed by reducing the initial system of secular equations into an effective N × N-dimensional secular problem, wherein a single equation corresponds to each of N elementary fragments of the initial chain. An implicit form of the dispersion relation and the level density function follow directly from the reduced problem without passing into the delocalized description of the system. The resulting eigenfunctions of the matrix H prove to be expressed as the Bloch sums of N nonorthogonal eigenvalue-dependent local-structure-determined orbitals of algebraic form, each of them corresponding to a definite elementary fragment of the chain. © 1996 John Wiley & Sons, Inc.
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    Increased-valence structures and hypervalent molecules (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 553-566 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hypervalent molecules may involve the use of increased-valence structures to provide valence bond descriptions of their electronic structure. For electron-rich molecules with four electrons distributed among three overlapping (nuclear-centered) atomic orbitals, the increased-valence structures are Y(BOND)A·B and Y·A -  - B. Each structure involves a fractional electron-pair bond and a one-electron bond. It is deduced that the Armstrong-Perkins-Stewart valence of the A atom is able to exceed unity in each of these structures when the three bonding electrons occupy nonorthogonal localized molecular orbitals. It is also shown that increased valence for the A atom does not occur when the four electrons occupy localized molecular orbitals to give the valence-bond structure Y - A - B with three overlapping atomic orbitals, and the same number of orbital variational parameters as occurs in the wave functions for either of the increased-valence structures. The results of ab initio valence bond calculations with minimal basis sets are reported for H-3l, CH-5, HF-2, F-3, CIF3, and FF3, and the resulting wave functions for resonance between six canonical Lewis structures are related to those for resonance between the two increased-valence structures. The use of the latter structures to indicate how electronic reorganization proceeds via one-electron delocalizations for SN2 reactions is redescribed, and an elementary argument is presented to deduce that this class of reactions cannot involve the delocalization of a pair of electrons in concert from the nucleophile. Increased-valence wave functions are used to deduce an expression for the avoided crossing for the transition state of the identity SN2 reaction. © 1996 John Wiley & Sons, Inc.
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  • 191
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    A comparison of Hartree - Fock, MP2, and DFT results for the HCN dimer and crystal (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 767-778 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of hydrogen-bond related quantities - geometries, interaction energies, dipole moments, dipole moment derivatives, and harmonic vibrational frequencies - were calculated at the Hartree - Fock, MP2, and different DFT levels for the HCN dimer and the periodic HCN crystal. The crystal calculations were performed with the Hartree - Fock program CRYSTAL92, which routinely allows an a posteriori electron-correlation correction of the Hartree - Fock obtained lattice energy using different correlation-only functionals. Here, we have gone beyond this procedure by also calculating the electron-correlation energy correction during the structure optimization, i.e., after each CRYSTAL92 Hartree - Fock energy evaluation, the a posteriori density functional scheme was applied. In a similar manner, we optimized the crystal structure at the MP2 level, i.e., for each Hartree - Fock CRYSTAL92 energy evaluation, an MP2 correction was performed by summing the MP2 pair contributions from all HCN molecules within a specified cutoff distance. The crystal cell parameters are best reproduced at the Hartree - Fock and the nongradient-corrected HF + LDA and HF + VWN levels. The BSSE-corrected MP2 method and the HF + P91, HF + LDA, and HF + VWN methods give lattice energies in close agreement with the ZPE-corrected experimental lattice energy. The (HCN)2 dimer properties are best reproduced at the MP2 level, at the gradient-corrected DFT levels, and with the B3LYP and BHHLYP methods. © 1996 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
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  • 192
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    Electronic Resource
    Valence energy in variational Monte Carlo: CuH dissociation energy (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 803-808 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We show how to estimate the dissociation energy of CuH using the variational Monte Carlo method. The techniques involved are (i) an all-electron approach, (ii) a diffusion-only Metroplis algorithm which is well-suited for sampling the nodal regions properly, and (iii) a core-valence partitioning scheme such that the dissociation energy is estimated from the valence energies of CuH and Cu only. This approach avoids several of the approximations inherent in pseudopotential methods. Using relatively crude wave functions, we obtain an estimate of the dissociation energy and dipole moment with an accuracy on par with much more elaborate calculations in the literature. © 1996 John Wiley & Sons, Inc.
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  • 193
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    Electronic Resource
    Basis sets for ab initio periodic Hartree - Fock studies of zeolite/adsorbate interactions: He, Ne, and Ar in silica sodalite (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 809-820 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Silica sodalite is an ideal model system to establish base-line computer requirements of ab initio periodic Hartree-Fock (PHF) calculations of zeolites. In this article, we investigate the effect of various basis sets on the structural and electronic properties of bulk silica sodalite. We also study the interaction of He, Ne, and Ar with the sodalite cage. Our work shows that basis-set superposition errors (BSSE) in calculations using STO-3G and 6-21G(*) basis sets are as large as the interaction energies, leading to poor confidence in the results. To cure this problem, we present high-quality basis sets for Si, O. He, Ne, and Ar, optimized for use with PHF methods, and demonstrate that the new basis set greatly reduces BSSE. The theoretical barriers for transfer of the rare gases between sodalite cages are 5.6, 13.2, and 62.1 kcal/mol for He, Ne, and Ar. © 1996 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
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  • 194
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    Electronic Resource
    Symmetry reduction of the matrix elements of a two-particle operator (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 833-841 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Local coordinate systems are chosen for each quadruple of atoms relative to a four-center integral, in order to avoid linear combinations of orbitals when symmetry operations perform on an orbital. This choice can utilize the complete molecular symmetry to attain the optimal number of symmetry-unique integrals and to construct two-particle matrix elements by multiplying symmetry-unique integrals, called the “standard four-center integrals,” by the corresponding coefficients, called the “C coefficients.” A simple algorithm to use the complete molecular symmetry to reduce calculations of molecular matrix elements is outlined for general highly symmetric molecules. A tetrahedral molecule is analyzed. © 1996 John Wiley & Sons, Inc.
    Additional Material: 3 Tab.
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  • 195
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    Analytical expressions for the count of LM-conjugated circuits of benzenoid hydrocarbons (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 943-958 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A recursive method for enumeration of linearly independent and minimal conjugated circuits of benzenoid hydrocarbons had previously been given which is valid for several classes of benzenoid hydrocarbons. In the present article, the properties and constructions of unique minimal conjugated circuits and pairs of minimal conjugated circuits of a ring s in a benzenoid hydrocarbon B are investigated. An analytical expression for the count of LM-conjugated circuits of B is given which is based on the counts of Kekulé structures of selected subgraphs of B. By using the method, the LMC expression of any benzenoid hydrocarbon can be obtained. © 1996 John Wiley & Sons, Inc.
    Additional Material: 16 Ill.
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  • 196
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    Electronic Resource
    Spin-free approach for evaluation of electronic matrix elements using character operators of ℒN (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 983-991 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A spin-free method is presented for evaluating electronic matrix elements over a spin-independent many-electron Hamiltonian. The spin-adapted basis of configuration state functions is obtained using a nonorthogonal spin basis consisting of projected spin eigenfunctions. The general expressions for the matrix elements are given explicitly, and it is demonstrated how the matrix elements may be obtained simply from the knowledge of the irreducible characters of the permutation group ℒN. The presented formulas are very general and may be applied in connection with both spin-coupled valence bond studies and in conventional configuration interaction (CI) methods based on an orthonormal orbital basis. © 1996 John Wiley & Sons, Inc.
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  • 197
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    Electronic Resource
    Selection of basis sets for atoms and molecules in cavities (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 993-999 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method of selection of basis functions for the quantum chemical description of atoms and molecules inside spherical and cuboidal cavities is presented. The method consists of two independent steps. First, the appropriate GTO or STO exponents are chosen by fitting the basis functions to cutoff orbitals. The criterion of maximal orbital density inside the cavity is next applied to form the final model orbital space. The method presents the fundamental advantage over many other methods of being trivially applicable in standard quantum chemical progam packages. As an illustration, the method is applied to the hydrogen atom in a spherical box. The use of contracted basis functions is also discussed. © 1996 John Wiley & Sons, Inc.
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  • 198
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    Electronic Resource
    Modifications of virtual orbitals in the limited CI calculations for electron-rich molecules (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1027-1036 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The efficiency of modified virtual orbitals (MVO) of ionic type and of approximate orthogonalized natural orbitals (ONO) in the CI-SD calculations was studied for O3 and SO2 molecules and compared with the commonly used canonical virtual orbitals (CVOs). The systems studied represent a class of electron-rich molecules, in which the number of valence electron pairs exceeds substantially the number of formal chemical bonds. We found that the modified orbitals of the types studied appear to be less effective for these systems than in the similar calculations for the AHn type molecules. Physical reasons for this difference were discussed. The evolution of spatial properties of virtual orbitals within the modification process was analyzed. © 1996 John Wiley & Sons, Inc.
    Additional Material: 9 Ill.
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  • 199
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    Electronic Resource
    Role of the oxygen content in TlBa1.2 La0.8 CuO5 + δ superconductors (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1057-1064 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic energy band structures of the La-doped superconductors TlBa1.2 La0.8 CuO5 + δ were calculated. The effect of the oxygen content on their electronic structures was studied. The results show that compared with those for TlBa2CuO5, the La doping at the Ba site results in the moderate change in the band structures and the decrease in the densities of states, but the increase in the oxygen content caused by the La doping results in the great change in the band structures and the densities of states near the Fermi surface. The low oxygen content causes the degree of complexity of the band structures and the densities of states near the Fermi level Ef to increase and the high oxygen content causes them to decrease. The oxygen content plays a dominant role in TlBa1.2La0.8CuO5 + δ. In addition, the Cu-O planes are most sensitive to the increase in the oxygen content. © 1996 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
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  • 200
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    Electronic Resource
    Application of localized molecular orbitals to the solution of semiempirical self-consistent field equations (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 133-146 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: When conventional matrix algebra is used to solve the semiempirical self-consistent field equations for large systems, the time required rises as the third power of the size of the system. A consequence of this is that self-consistent calculations of large systems such as enzymes are impractical. By using localized molecular orbitals instead of matrix methods, the time required for these systems can be made almost proportional to the size of the system. In partial geometry optimizations, the time required depends only upon the size of the fragment being optimized and is almost independent of the size of the whole system. © 1996 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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