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  • 1995-1999  (370)
  • 1995  (370)
  • Atomic, Molecular and Optical Physics  (370)
  • Nuclear reactions
  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 361-374 
    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: All possible quadratic factors from the characteristic polynomial are generated. Selective extraction of the factors are smoothly achieved. Factors of the mirror plane and two-, three-, and n-fold rotationally symmetric graphs are obtained without using any rule for the fragmentation pattern. The relationship among the irreducible representations of the two-, three-, and n-fold operators are established through our computer program. © 1995 John Wiley & Sons, Inc.
    Additional Material: 4 Tab.
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  • 2
    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 present article develops a methodology and a unifying theorem to treat, on an equal footing, mathematical phenomena that were hitherto studied separately in each of the research fields of dynamical systems and quantum chemistry involving the spectral symmetry of alternant hydrocarbons. This article also serves as a foundation of a theoretical framework for the analysis of certain dynamical systems of chemical kinetic equations, which shall be made in the context of operator algebra in Parts II and III of this series of papers. © 1995 John Wiley & Sons, Inc.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 583-590 
    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 efficient procedure for third-order electron propagator calculations of ionization energies and electron affinities is reported. Diagonal self-energy expressions that are suitable for large molecules are empolyed. The outer-valence Green's function method also is implemented. An integral transformation program for direct and semidirect algorithms is modified to store only nonzero integrals according to Abelian point group symmetry. Contributions to self-energy matrix elements that depend on electron repulsion integrals with four virtual orbital indices are computed in a direct way. Intermediate batches of integrals are created by sort procedures while avoiding storage of transformed integrals in the main memory. This method permits calculation of electron binding energies for C72- with a 231 atomic orbital basis and for Zn(C5H5)2 with a 220 atomic orbital basis on an IBM RISC/6000 Model 550. During these calculations, the CPU is engaged approximately 90% of the time. © 1995 John Wiley & Sons, Inc.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 591-606 
    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 article, we derive and there by reinterpret various approximations in Schrödinger theory and Kohn-Sham density-funtional theory via a hierarchy within the work formalism of electronic structure due to Harbola and Sahni. In the work formalism, which is based on Coulomb's law, the local potential representing electron correlations as well as the electron correlation energy both arise from the same quantum mechanical source charge distribution that is the pair-correlation density. The potential is the work done to move an electron in the force field of the pair-correlation density, and the energy is the energy of interaction between the electronic and pair-correlation densities. The differential equation governing the system is a sturm-Liouville equation so that the system wave function can, in principle, be obtained as an infinite linear combination of Slater determinants of the spin-orbitals corresponding to the occupied and virtual states. The hierarchy is achieved by improvement of the pair-correlation density either by systematic improvement of the wave function or, as is the case of Kohn-Sham theory, by an expansion of the pair-correlation density in gradients of the density about the uniform electron gas result. The derivations of the approximations of Kohn-Sham theory via the work formalism, in turn, exhibit the existence of additional correlations that are not evident through the Kohn-Sham prescription, whereby the potential is obtained by functional differentiation. The approximations considered within Schrödinger theory are the Hartree, Hartree-Fock, and configuration-interaction approximations. Those within Kohn-Sham theory are the density functional theory Hartree, local density, and gradient expansion approximations. © 1995 John Wiley & Sons, Inc.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 617-625 
    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 alkylation of arylamines is an important reaction in many large scale industrial processes. Three possible mechanisms are evaluated at the semiempirical level using the alkylation of aniline with ethylene as a model system. A molecular orbital analysis is performed for each mechanism to determine the appropriate type of wavefunction required to describe the transition state. Successive levels of refinement in search algorithms are used to locate the transition state on a potential surface with up to 60 degrees of freedom. A reasonable mechanism for the AlCl3-catalyzed Friedel-Crafts alkylation is shown to be analogous to a Diels-Alder [4 + 2] concerted cycloaddition after initial β-elimination of HCl. The Brönsted acid-catalyzed reaction shown is likely to occur via a mechanism similar to the Ene reaction, rather than initial protonation of the olefin followed by electrophilic addition to a nearby aromatic ring. © 1995 John Wiley & Sons, Inc.
    Additional Material: 12 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 43-49 
    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 structures of P2 + P2+ were studied with ab initio calculations at the ROHF and UHF levels with the 6-31G* basis set. The geometries and dissociation energies for the four selected structures-collinear, T-shaped, regular trapezoid, and elongated tetrahedral-were studied in comparison with N2 + N2+. The trade-off of the intramolecular π bond for the intermolecular σ bond for the P4+ system results in its larger dissociation energies and more substantial changes in bond distances than those in the N4+ system. © 1995 John Wiley & Sons, Inc.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 1-2 
    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
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 19-26 
    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 concept of conjugate irreducible subspaces in the algebras of permutation groups is expressed in such a way that it emphasizes the existence of conjugate pathways along a chain of subgroups of permutations. The new expression is shown to be equivalent to parity conjugation and incorporates the notion of conjugate pathways in a natural and consistent manner. The example of the Aufbau chain of symmetric subgroups is used to show how the conjugacy of pathways may be useful in the symmetry adaptation of the spatial and spin parts of Fermion state functions. It is also employed to show that one may arrive at a quantitative statement that clearly expresses the conjugacy constraints placed upon a Fermion state function in order for it to satisfy the antisymmetry principle. A simple example of these conjugacy conditions is presented. © 1995 John Wiley & Sons, Inc.
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  • 9
    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: Several types of approximations have been used for the justification of the Neglect of Diatomic Differential Overlap (NDDO) in Part IV but control of the introduced error remains insufficient. Analytic formulas describing the induced error for all types of approximations are given. Numerically lower bounds for these errors can be derived from the discussion on diatomic molecules. Far-reaching consequences on the applicability of NDDO will be discussed. © 1995 John Wiley & Sons, Inc.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 575-582 
    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: Atomic Hylleraas-CI calculations with linked correlation terms of the form rijp rklq are discussed. Formulas for the integration of the radial part and the arising auxiliary integrals are deduced and convergence proofs are given. © 1995 John Wiley & Sons, Inc.
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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995) 
    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
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 197-199 
    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 Neumann expansion for 1/r12 in elliptical coordinates a combined formula has been obtained for two-center Coulomb, hybrid, and exchange integrals with Slater-type orbitals. © 1995 John Wiley & Sons, Inc.
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  • 13
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 201-210 
    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 to obtain ensemble representable densities from experimental diffraction data is proposed. The method uses ab initio molecular densities instead of the commonly employed one-electron orbital densities, and as a result, few parameters need to be optimized in the fitting procedure to the experimental structure factors. The optimized coefficients can provide information about intra- and intermolecular electronic correlations, spin-orbit coupling, etc. This work also provides new explicit formulas to determine the rank of a fermionic wave function, i.e., the rank of the one-fermion density matrix. © 1995 John Wiley & Sons, Inc.
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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 55 (1995), S. 315-328 
    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 effective core potential study of a model chemical vapor deposition (CVD) reaction is reported. Compounds studied are those in which a Group IVA (E = C, Si, Ge, Sn) main group element is directly bonded to a Group IVB (M = Ti, Zr, Hf) transition metal. Model reactants (H3M - EH3) possess Group IVA-Group IVB single bonds while products (H2M = EH2, formed by H2 elimination from reactants) have Group IVA-Group IVB multiple bonds. The main findings of this research are as follows: First, a single-determinant (RHF) description is appropriate for singly bonded Group IVA-Group IVB complexes. Agreement between experimental and calculated M-E single-bond lengths is very good at all levels studied. Second, electron correlation is of much greater importance for describing the ME π bond than the ME σ bond. Third, analysis of calculated double-to-single bond ratios (Rdouble/Rsingle) suggest that π-bonding remains nearly constant over the entire series of compounds studied and is weaker than in the main group-main group analogs, suggesting a reason for the lack of reported examples with a Group IVA-Group IVB double bound. Fourth, barriers to elimination of H2 from H3M - EH3 show two significant trends. We propose that the trends as a function of transition metal (Ti 〈 Zr 〈 Hf) and main group element (C 〉 Si 〉 Ge 〉 Sn) are due to trends in M - H and E - H bond energies. © 1995 John Wiley & Sons, Inc.
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 211-222 
    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 reduction of the N-particle problem to the 2- and 1-particle problems is discussed, and the relation between the statistics of particles and the properties of the pairon distribution is studied. It turns out that not all levels of the pairon spectra can be occupied. A simple way of defining the pairon distribution is suggested. The spectra of the reduced Hamiltonian with power-law interaction potentials are analyzed. The bound states are calculated by the method of self-similar approximation, the accuracy of which is compared with and shown to be better than that of the quasi-classical approximation. © 1995 John Wiley & Sons, Inc.
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 243-247 
    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: Based on the conformation dynamics of macromolecules, the rate of conformational transition is deduced from the nonadiabaticity operator method which can be used to explain the time scale of milliseconds for protein folding. It is proved that (1) the dependence of the transition rate on inertial moment I of the atomic group obeys the I-2.5 law; (2) its dependence on numbers n of torsional angles participating in the transition obeys the n1.5 law; and (3) the temperature dependence of the transitional rate shows an abnormal character in the high- temperature region. © 1995 John Wiley & Sons, Inc.
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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 261-263 
    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: Accurate nonrelativistic numerical Hartree-Fock results are reported for the heavy atoms Cs (Z = 55) through Lr (Z = 103) in their ground states. © 1995 John Wiley & Sons, Inc.
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  • 18
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 265-270 
    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 three-dimensional EHMO crystal orbital calculations for crystalline C60, Ca3C60, and Ca5C60 are reported. The ground states of both undoped solid C60 and partially doped Ca3C60 are found to be insulating with an indirect energy gap of 1.2 and 0.5 eV, respectively. In contrast, Ca5C60 forms a metallic conducting phase with a set of three half-filled bands crossing the Fermi level, which is found to be located close to a peak of the density of state. The character of crystal orbitals near the Fermi level for both Ca3C60 and Ca5C60 is completely carbonlike. In both cases, the Ca atoms are almost fully ionized and C60 molecules form a stable negative charge state with six to 10 additional electrons. © 1995 John Wiley & Sons, Inc.
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  • 19
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 1-2 
    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
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995) 
    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
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  • 21
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 351-359 
    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 derivation of hybrids as localized equivalent functions in the plane is discussed using the simultaneous eigenfunctions of the x and y position operators, as represented in a finite basis. It proves helpful, initially, to use complex exponentials as basis functions, but the transformation to a real basis is made later. The introduction of alias functions to produce commuting matrices is described. Full results are obtained for any number of functions in the plane. © 1995 John Wiley & Sons, Inc.
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  • 22
    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 effects of quartic anharmonicity on the quantum dynamics of a linear oscillator with time-dependent force constant (K) or harmonic frequency (ω) are studied both perturbatively and numerically by the time-dependent Fourier grid Hamiltonian method. In the absence of anharmonicity, the ground-state population decreases and the population of an accessible excited state (k = 2, 4, 6…) increases with time. However, when anharmonicity is introduced, both the ground- and excited-state populations show typical oscillations. For weak coupling, the population of an accessible excited state at a certain instant of time (short) turns out to be a parabolic function of the anharmonic coupling constant (λ), when all other parameters of the system are kept fixed. This parabolic nature of the excited-state population vs. the λ profile is independent of the specific form of the time dependence of the force constant, Kt. However, it depends upon the rate at which Kt relaxes. For small anharmonic coupling strength and short time scales, the numerical results corroborate expectations based on the first-order time-dependent perturbative analysis, using a suitably repartitioned Hamiltonian that makes H0 time-independent. Some of the possible experimental implications of our observations are analyzed, especially in relation to intensity oscillations observed in some charge-transfer spectra in systems in which the dephasing rates are comparable with the time scale of the electron transfer. © 1995 John Wiley & Sons, Inc.
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  • 23
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 27-35 
    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: By means of examples, Löwdin showed that L2 convergence of approximate wave functions ψn to the exact ψ using the single limit limn→∞〈ψn, Aψn〉 = 〈ψ,Aψ〉 is not sufficient to compute accurate expectation values. It is shown that L2 convergence is indeed a sufficient condition to compute accurate expectation values using iterated limits limm→∝ limn→∝〈ψn, Aψm〉 = 〈ψ, Aψ〉 instead of a single limit. Practical conditions that guarantee the stability of single-limit calculations are given. It is also shown that the L2 covergence of wave functions implies the convergence in the L1(R3)-norm of their corresponding densities. This permits us to prove Weinhold's conjecture that the rate of convergence of densities are greater than that of wave functions. The results are extended to the momentum space, and their equivalence with those of position space is shown. Properties of Lp spaces are used to introduce the Cauchy criterion that permits us to check the convergence in norm of approximate wave functions and their densities, as well as to estimate exact errors. This is illustrated by a numerical example. © 1995 John Wiley & Sons, Inc.
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  • 24
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 49-56 
    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 geometries, successive binding energies, vibrational frequencies, and infrared intensities are calculated for the [Li(H2O)n]+ and [K(H2O)n]+ (n = 1-4) complexes. The basis sets used are 6-31G* and LANL1DZ (Los Alamos ECP+DZ) at the SCF and MP2 levels. There is an agreement for calculated structures and frequencies between the MP2/6-31G* and MP2/LANL1DZ basis sets, which indicates that the latter can be used for calculations of water complexes with heavier ions. Our results are in a reasonable agreement with available experimental data and facilitate experimental study of these complexes. © 1995 John Wiley & Sons, Inc.
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  • 25
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 57-65 
    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 is shown that the mechanism of spontaneous infrared emission enhances substantially the stability of the long-lived ground electronic state of the exotic BeH2+, whose autodissociation becomes possible only via tunneling. The system, initially situated in any vibrational level (except the high-lying ones for which dissociation is predominate) and statistically distributed over all rotational states, reaches stability through a cascade of dipole emissions toward lower levels. The rapid spatial variation of the dipole moment results in fast radiative processes and might suggest experimental observations easier to perform. The methodology is based on a recently presented analytic discrete variable representation (DVR) [D.T. Colbert and W.H. Miller, J. Chem. Phys. 96, 1982 (1992)] that facilitates calculations. This might prove particularly useful to spectroscopists, allowing a check of the quality of potentials produced by inversion methods or the reliable calculation of spectrochemical quantities. © 1995 John Wiley & Sons, Inc.
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  • 26
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    International Journal of Quantum Chemistry 53 (1995), S. 37-47 
    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 stepwise acid dissociation constants for p-benzohydroquinone (QH2) in aqueous media have been explicitly calculated for the first time, with the INDO parametrized SCF-MO method. We have optimized the geometries of QH2, QH-, and Q2- and of the QH2 · 6H2O, QH- · (H3O+) · 5H2O, and Q2- · (H3O+)2 · 4H2O systems that model the solvated species. The presence of the associated water molecules (and hydronium ions) account for the stabilization due to hydrogen bonding as well as for a part of the effect of interaction of these molecules with the respective reaction fields in an aqueous medium. To simulate the first solvation shell in a more complete manner, four more water molecules have been considered to be placed above and below the quinonoid ring and the optimized geometries of the resulting hydrated species, QH2 · 10H2O, QH- · (H3O+) · 9H2O, and QH- · (H3O+) · 8H2O, have been determined. The standard free-energy changes calculated for the dissociation of QH2 into QH- and H+ is 0.0251 Hartree (65.9 kJ mol-1) and that of QH- into Q2- and H+ is 0.0285 Hartree (74.8 kJ mol-1). Experimentally observed dissociation constants for these two steps correspond to free-energy changes of 0.0214 Hartree (56.2 kJ mol-1) and 0.0248 Hartree (65.1 kJ mol-1), respectively. © 1995 John Wiley & Sons, Inc.
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  • 27
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    International Journal of Quantum Chemistry 53 (1995), S. 67-76 
    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 consider a crystal as partitioned into a localized molecular cluster (containing a defect or not) and an embedding region. Within the Hartree-Fock formalism, an expression is derived for an effective potential due to the embedding region of crystal. This potential is part of the cluster Fock operator and requires input from a perfect crystal calculation. Special features of the derivative are rigorous inclusion of cluster-embedding overlap and orthogonality between single-electron states of the embedding region and the function-space manifold of the cluster; physically correct normalization of the Fock eigenstates; and a nontrivial total-energy algorithm. Computational requirements are qualitatively compared with those for an isolated cluster. The method allows for intracluster (and intraembedding) correlation and can be adapted straightforwardly to local density functional approaches. Fundamental aspects of the embedding problem are addressed in a general formulation that is, nevertheless, oriented toward explicit calculations. © 1995 John Wiley & Sons, Inc.
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  • 28
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    International Journal of Quantum Chemistry 53 (1995), S. 77-81 
    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 report here the first quantitative theory of the change of anisotropic Fermi surfaces (FS) due to the dynamical e-e interaction. The new FS is constructed as a self-consistent solution of the Dyson equation. This incorporates effects of “anomalous” diagrams, absent in the usual perturbation theory, but which are responsible for changing the FS. Calculations are presented for 2D electrons. Correlations reduce the anisotropy. © 1995 John Wiley & Sons, Inc.
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  • 29
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    International Journal of Quantum Chemistry 53 (1995), S. 95-95 
    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
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  • 30
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995) 
    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
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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 theoretical study of the thermally allowed conrotatory opening of cyclobutene (1) and cis- (2) and trans-1,2-dihydro-1,2-diazacyclobutadiene (3) were performed with ab initio and density functional calculations. The reactants and the transition states were fully optimized by using the 6-31 + G** basis set with RHF, MP2, SVWN, and BLYP methods. The calculated activation barriers for the ring opening of 1 with both MP2 and SVWN incorporating ZPVF correction give extraordinary agreement with the experimental value. The predicted activation energies for 2 and 3 are lower than in the case of the cyclobutene ring opening. Of the two 1,2-dihydro-1,2-diazacyclobutadiene isomers, the trans isomer has a lower activation barrier. The structural and energy differences and the trend among these compounds are interpreted in terms of orbital overlap and steric interactions in the course of the conrotatory ring opening. © 1995 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 56 (1995), S. 157-162 
    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 relationship between the atomic form factor, F(k), and its first derivative, both at the origin, is presented. A new function f(k), related in a simple way to F(k), has been studied and some applications have been performed. They led us to find lower bounds to F(k) for all k and to other quantities such as the charge density at the origin, ρ(0), and radial expectation values. Finally, interesting effects on the Laplacian of f(k) due to the atomic shell structure were found. © 1995 John Wiley & Sons, Inc.
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  • 33
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    International Journal of Quantum Chemistry 56 (1995), S. 163-173 
    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 automated method by the algebraic programming language REDUCE3 for specifying the matrix elements expressed in second quantization language is presented and then applied to the case of the matrix elements in the TDHF theory. This program works in a very straightforward way by commuting the electron creation and annihilation operators (a† and a) until these operators have completely vanished from the expression of the matrix element under the appropriate elimination conditions. An improved method using singlet generators of unitary transformations in the place of the electron creation and annihilation operators is also presented. This improvement reduces the time and memory required for the calculation. These methods will make programming in the field of quantum chemistry much easier. © 1995 John Wiley & Sons, Inc.
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  • 34
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    International Journal of Quantum Chemistry 56 (1995), S. 175-186 
    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 work deals with two shortcomings in the use of Morse potentials to describe energy spectra and transitions of diatomic molecules: (1) Morse's well-known “exact” solution for purely vibrational states includes the unphysical region - ∞ 〈 r 〈 0 of the internuclear separation, and (2) Franck-Condon factors are evaluated in harmonic and anharmonic approximations to the Morse potentials. The method of confining the molecule in a spherical box is developed to obtain (1) purely vibrational energy spectra and eigenvectors of Morse potentials in the physical region 0 ≤ r 〈 ∞, and (2) the corresponding Franck-Condon factors without any additional approximations. The method is applied to Li2, N2, CN, and CO molecules. © 1995 John Wiley & Sons, Inc.
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  • 35
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    International Journal of Quantum Chemistry 56 (1995), S. 129-155 
    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 that the irreducible tensor operators of the unitary group provide a natural operator basis for the exponential Ansatz which preserves the spin symmetry of the reference state, requires a minimal number of independent cluster amplitudes for each substitution order, and guarantees the invariance of the correlation energy under unitary transformations of core, open-shell, and virtual orbitals. When acting on the closed-shell reference state with nc doubly occupied and nv unoccupied (virtual) orbitals, the irreducible tensor operators of the group U(nc) ⊗ U(nV) generate all Gelfand-Tsetlin (GT) states corresponding to appropriate irreducible representation of U(nc + nv). The tensor operators generating the M-tuply excited states are easily constructed by symmetrizing products of M unitary group generators with the Wigner operators of the symmetric group SM. This provides an alternative to the Nagel-Moshinsky construction of the GT basis. Since the corresponding cluster amplitudes, which are also U(nc) ⊗ U(ns) tensors, can be shown to be connected, the irreducible tensor operators of U(nc) ⊗ U(nv) represent a convenient basis for a spin-adapted full coupled cluster calculation for closed-shell systems. For a high-spin reference determinant with n, singly occupied open-shell orbitals, the corresponding representation of U(n), n=nc + nv + ns is not simply reducible under the group U(nc) ⊗ U(ns) ⊗ U(nv). The multiplicity problem is resolved using the group chain U(n) ⊃ U(nc + nv) ⊗ U(ns) ⊃ U(nc) ⊗U(ns)⊗ U(nv) ⊗ U(nv). The labeling of the resulting configuration-state functions (which, in general, are not GT states when nc 〉 1) by the irreducible representations of the intermediate group U(nc + nv) ⊗U(ns) turns out to be equivalent to the classification based on the order of interaction with the reference state. The irreducible tensor operators defined by the above chain and corresponding to single, double, and triple substitutions from the first-, second-, and third-order interacting spaces are explicitly constructed from the U(n) generators. The connectedness of the corresponding cluster amplitudes and, consequently, the size extensivity of the resulting spin-adapted open-shell coupled cluster theory are proved using group theoretical arguments. The perturbation expansion of the resulting coupled cluster equations leads to an explicitly connected form of the spin-restricted open-shell many-body perturbation theory. Approximation schemes leading to manageable computational procedures are proposed and their relation to perturbation theory is discussed. © 1995 John Wiley & Sons, Inc.
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  • 36
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    International Journal of Quantum Chemistry 56 (1995) 
    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
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  • 37
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    International Journal of Quantum Chemistry 56 (1995), S. 187-195 
    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: Based on the EHMO approach, the band structures for the Y—Ba—Cu—O superconductors doped by La were calculated. The influence of the partial substitutions of La for Y and Ba in YBa2CU3Oy on its electronic structures was investigated. The results demonstrate that the La doping at the Ba site has a great effect on the electronic structures of the Y—Ba—Cu—O superconductors, whereas the change in the band structures caused by the La doping at the Y site is very small. The increase in the oxygen content caused by the La doping results in an increase in the densities of states at Ef, N(Ef), for La1+x Ba2-xCu3Oy, but the increase in N(Ef) cannot compensate the decrease caused by the La doping at the Ba site. In addition, the 2D Cu—O planes are much more sensitive to the change in N(Ef) than are the 1D Cu—O ribbons, which implies an important role of the 2D Cu—0 planes in the Y—Ba—Cu—O superconducting system, regardless of whether La substitutes for Y or for Ba. © 1995 John Wiley & Sons, Inc.
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  • 38
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    International Journal of Quantum Chemistry 56 (1995), S. 197-198 
    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
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  • 39
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    International Journal of Quantum Chemistry 56 (1995), S. 211-215 
    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 proof of Parr and Chattaraj for the Principle of Maximum Chemical Hardness is very general and can be applied to many more observables. The case of the physical, or mechanical, hardness of a solid is taken as an example. It is shown that this also should be a maximum in an equilibrium system. Assuming the validity of the argument leads to new information about the compressibility of solids. A simple expression is also given for the Gruneisen constant. These maximum principles are necessary consequences of the equilibrium conditions of quantum mechanics and thermodynamics. © 1995 John Wiley & Sons, Inc.
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  • 40
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    International Journal of Quantum Chemistry 56 (1995), S. 217-223 
    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 field theoretical background of relativistic density functional theory is emphasized and its consequences for relativistic Kohn-Sham equations are shown. The local density approximation for the exchange energy functional is reviewed and the importance of relativistic corrections for an accurate representation of the exchange functional is demonstrated. © 1995 John Wiley & Sons, Inc.
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  • 41
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    International Journal of Quantum Chemistry 53 (1995), S. 161-181 
    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 relativistic Kepler problem is discussed, with emphasis on the exact supersymmetry of the problem. It is shown that the supersymmetry is generated by the Johnson-Lippmann operator. Two related operators are found to generate new supersymmetries in an extended function space. Each of these supersymmetries may be disguised as radial supersymmetries. The radial supersymmetries are discussed and it is shown that each of them defines a normal-mode representation of the hydrogen-atom radial functions. Thus, one obtains two different, but equivalent, analytical expressions for these functions. The expressions are well known, but are rederived here in the light of the new understanding. Finally, the nonrelativistic image of the relativistic supersymmetry is constructed and its generators shown to be identical with those recently presented in the literature. © 1995 John Wiley & Sons, Inc.
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  • 42
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    International Journal of Quantum Chemistry 53 (1995), S. 183-187 
    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, we have attempted a systematic procedure for use of biorthogonal techniques to the configuration interaction studies in molecules using nonorthogonal valence bond (VB) orbitals. The procedure developed is integral-driven and a program based on this has been developed. Test runs of the program have been carried out in case of full and truncated configuration spaces. © 1995 John Wiley & Sons, Inc.
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  • 43
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    International Journal of Quantum Chemistry 53 (1995), S. 207-228 
    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 the quasi-degenerate many-body perturbation theory up to the third order is investigated for the ground state, five excited states, and the first quintet of a simple four-electron H4 model system consisting of two stretched hydrogen molecules, in which the degree of quasi-degeneracy can be continuously varied from a nondegenerate to a full degenerate situation. We employ a DZP basis set. The effect of intruder states is considered and a comparison with other multireference correlation techniques is also provided. Finally, a criterion for the model space to be quasi-degenerate will be reformulated and generalized. © 1995 John Wiley & Sons, Inc.
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  • 44
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    International Journal of Quantum Chemistry 53 (1995), S. 189-205 
    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 article introduces a set of localized orthonormal functions to serve as basis functions for quantum calculations. They are defined to be eigenfunctions of the position operator in a function space. Their properties, including their variances, for a one-dimensional system are developed. The application to simple harmonic motion is considered as an example and, in particular, the time evolution of an initially localized function is calculated and shown to be periodic. The theory can be interpreted as producing a discrete quantization of space with Hamiltonian interactions that are predominantly between nearest neighbors. These functions can also be used in approximate calculations. To illustrate their accuracy, the example of a Morse oscillator treated as a perturbation of a harmonic oscillator is reconsidered. It is shown that the localized functions in a variational calculation lead to a result that is a good approximation for the lowest states. Furthermore, the use of a wave function that is defined only at discrete points can be justified as the first approximation to this, so that its accuracy can also be discussed. © 1995 John Wiley & Sons, Inc.
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  • 45
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    International Journal of Quantum Chemistry 56 (1995), S. 239-246 
    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 stabilized jellium model is a simple modification of the jellium model, which more realistically describes simple metals of high density, such as Al, Ga, Pb, etc. We analyzed the fragmentation processes of charged spherical Al clusters in the framework of the stabilized jellium model. Kohn-Sham calculations of the parents and daughters, using the local density approximation, have been made. We evaluated the dissociation energies of AlN+, AlN2+, and AlN-, with N=1-30 atoms, in all possible decay channels. We discuss the most favorable decay channels, which are ruled by the shell structure (magic numbers of valence electrons in the parents and the daughters) oscillations around an average trend given by a liquid drop model. We compare our calculations with others and with the available experimental data. © 1995 John Wiley & Sons, Inc.
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  • 46
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    International Journal of Quantum Chemistry 56 (1995), S. 247-255 
    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 progress report is given of an extension of the density functional formalism to include long-range interactions such as van der Waals or dispersion forces. This is done by proving a general expression for the so-called exchange-correlation energy to contain and to describe such interactions just as well as any other treatment. The proper long-range forms of the interactions are derived explicitly in the cases of two neutral atoms, an atom outside a metal surface, and two parallel metal surfaces. The long-standing problem of treating the attractive and repulsive forces on the same footing in this way gets a solution. For practical calculations, an approximate form, based on an analysis in the weakly inhomogeneous limit and on a limiting form of the three-point function given by Rapcewicz and Ashcroft, is proposed and applied to some prototype cases. © 1995 John Wiley & Sons, Inc.
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  • 47
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    International Journal of Quantum Chemistry 56 (1995), S. 371-383 
    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 focal point of the present work is the single-particle kinetic energy density tensor in D dimensions. This quantity enters both differential and various integral forms of the virial theorems, which are again set up in D dimensions. Major new results lie in (i) demonstrating that, by one-dimensional quadrature, it is possible to construct the Pauli potential directly from the kinetic energy tensor, without the need for functional differentiation and (ii) generating the gradient expansion for the kinetic energy tensor, in D dimensions. © 1995 John Wiley & Sons, Inc.
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  • 48
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    International Journal of Quantum Chemistry 56 (1995), S. 385-388 
    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 the purpose of improving present approximations to the exchange and correlation potentials, newly derived properties of the exact exchange and correlation potentials are summarized. Present approximations are not expected to generally satisfy these properties. The summarized properties include relations at the Fermi level, low-density requirements, and a new density functional formula for computing ionization energies. © 1995 John Wiley & Sons, Inc.
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  • 49
    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: Experimentally measured rates for the oxidation of p-substituted benzyl amines by bovine monoamine oxidase type B (MAO-B) derived from the literature were examined with respect to the effects of molecular (semiempirically (AM1) derived) electronic, steric, and lipophilicity parameters. These properties included vertical and adiabatic ionization potential, LUMO energy, the LUMO-HOMO difference, molecular hardness, absolute electronegativity, calculated log P values, molecular volume, surface area, and ovality. Substrate oxidation rates (log kcat/Km) were found to correlate with molecular ovality and vertical ionization potential while the rate of enzymatic (flavin) reduction associated with substrate oxidation (log kred) was described by a two-parameter model containing an ovality and an absolute electronegativity term. These results are consistent with an initial one-electron substrate oxidation mechanism. In previous work, use of classical Hansch analysis suggested that electronic terms were not important in the enzymatic reactions. This discrepancy may be related to nontransferability inherent in fragment approaches which assume that the substituent of interest behaves similarly in all molecular scaffolds. Analysis of substrate binding (log Kd) to the enzyme was described by a two-parameter model containing a calculated log P term as well as LUMO energy. The significant correlation found with LUMO energy is consistent with studies suggesting that this property is important for drug-receptor interactions. © 1995 John Wiley & Sons, Inc.
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  • 50
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    International Journal of Quantum Chemistry 56 (1995), S. 201-206 
    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 integrated molecular transform (FTm) is a unitary structure index that has been successfully used for the correlation of 2- and 3-dimensional structure representations with their physicochemical and pharmacological properties. In the present instance the reported pKa values in a series of 30 compounds consisting of five subseries were correlated with their FTm indices. The omission of four outliers gave a moderate correlation across the entire series; within the individual subseries the correlations were considerably improved even with inclusion of the outliers. In general, the logarithmic transform of the pKa did not improve the correlations. This method gives a relatively simple means of estimating pKa in several structure classes. © 1995 John Wiley & Sons, Inc.
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  • 51
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    International Journal of Quantum Chemistry 56 (1995) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
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  • 52
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    International Journal of Quantum Chemistry 56 (1995), S. 207-225 
    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 Raman and infrared spectra of the Watson-Crick type of the guanine cytosine base pair and of the individual guanine and cytosine nucleic acid bases by ab initio Hartree-Fock theory using the 6-31G* basis set. IR and Raman intensities and Raman depolarization ratios were predicted using the double-harmonic approximation. The effects of a polar solvent were modeled by the self-consistent reaction field (SCRF) approximation. Variations in geometries, harmonic force constants, and vibrational spectra of the studied nucleobases due to the specific hydrogen-bonding interactions are discussed. © 1995 John Wiley & Sons, Inc.
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  • 53
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    International Journal of Quantum Chemistry 56 (1995), S. 791-799 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The possibility of an antiferromagnetic phase in a two-dimensional electron gas is investigated within the density functional formalism of phase transitions. © 1995 John Wiley & Sons, Inc.
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  • 54
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    International Journal of Quantum Chemistry 56 (1995), S. 787-790 
    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 the first full ab initio calculation of second-order Raman spectra in semiconductors based on density functional perturbation theory. The method is applied to the Γ1+ spectra of the elemental semiconductors diamond, silicon, and germanium and reproduces the experimental data excellently. Using first-principles phonons, we also calculated the corresponding overtone densities of states and Γ1+ Raman spectra employing phenomenological polarizability coefficients. Finally, we analyze the results of the different approaches pointing out the relevance of a full first-principles derivation. © 1995 John Wiley & Sons, Inc.
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  • 55
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    International Journal of Quantum Chemistry 56 (1995), S. 819-823 
    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 variation of the charge capacity of a double-layer 1T-TiS2 model system upon increasing intercalation with lithium is examined at a molecular level. The charge capacity is modeled through the global hardness index of density functional theory. The results obtained qualitatively reproduce the experimental trend observed in the voltage-composition variation curve in this system. Comparison with previous theoretical models show that the present approach may give a more complete information about the double-layer capacitance, since it takes into account the lithium ionicity and the host-guest specific interactions. © 1995 John Wiley & Sons, Inc.
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  • 56
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    International Journal of Quantum Chemistry 56 (1995), S. 801-817 
    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 first-principles calculations of the structural, lattice dynamical, and thermal properties as well as Raman results for cubic silicon carbide (3C SiC). The plane-wave pseudopotential approach to density functional theory (DFT) in the local density approximation has been used to calculate the equilibrium properties of 3C SiC, i.e., the ground-state energy, the band structure, the valence electron density, the lattice constant, the bulk modulus, its pressure derivative, and the ionicity factor of the chemical bonds. The linear-response theory within DFT has been used to obtain the phonon frequencies, the eigenvectors, and the mean-square atomic displacements. Furthermore, we calculated the mode Grueisen parameters, the internal-strain parameter, the elastic constants, the Born effective charge, and the high-frequency dielectric constant. The specific heat at constant volume and at constant pressure, the thermal expansion coefficient, the temperature dependence of the lattice constant, and that of the isothermal and adiabatic bulk modulus have been derived within the quasi-harmonic approximation. Finally, the second-order Raman spectrum of 3C SiC has been calculated using phenomenological polarizability coefficients and ab initio frequencies and eigenvectors. © 1995 John Wiley & Sons, Inc.
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  • 57
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    International Journal of Quantum Chemistry 56 (1995), S. 825-830 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ground-state energies of hydrogenic and helium chains in superstrong magnetic fields (B ≍ 107-109 Tesla) have been calculated within the single-particle scheme of a heuristic density functional method. Applying the local density approximation (LDA), the simple Dirac exchange functional was used neglecting correlation effects. The equations were solved iteratively using a basis set of Landau functions in the transverse directions and directly integrating along the longitudinal direction. We obtained binding lengths and, by comparing the binding energies of the chains with the ground-state energies of single atoms, we found condensation energies. © 1995 John Wiley & Sons, Inc.
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  • 58
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    International Journal of Quantum Chemistry 55 (1995), S. 127-132 
    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 coupled-cluster method is applied to high sectors of the Fock space (up to five electrons outside a closed shell). Ionization potentials and excitation energies are calculated for atomic N, O, F, P, and S and their ions. The effect of virtual triple excitations is included, exactly or at several levels of appoximation. These terms are important for accurate results in the high sectors. © 1995 John Wiley & Sons, Inc.
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  • 59
    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 orthogonally spin-adapted (OSA) coupled-pair (CCD) methods with an approximate account of triply and quadruply excited clusters are considered. We focus on the CCD + ST(CCD) perturbative estimate of the singly and triply excited clusters due to Raghavachari [J. Chem. Phys. 82, 4607 (1985)] and its ACPQ + ST(ACPQ) analog proposed by Paldus and Piecuch [Int. J. Quantum Chem. 42, 135 (1992)]. The latter approach combines the perturbative treatment of singles and triples with an approximate CCD theory corrected for connected quadruply excited clusters (ACPQ). We also consider the OSA version of the CCSD + T(CCSD) method (coupled-cluster [CC] approach with singly and doubly excited clusters and noniterative perturbative account of triply excited clusters) introduced by Urban et al. {J. Chem. Phys. 83, 4041 (1985)}. The explicit OSA expressions for the previously neglected {P. Piecuch and J. Paldus, Theor. Chim. Acta 78, 65 (1990)} S(CCD) and S(ACPQ) terms are derived using diagrammatic methods of many-body perturbation theory and graphical methods of spin algebras. The CCD + ST(CCD), CCSD + T(CCSD), and ACPQ + ST(ACPQ) formalisms have been implemented and the general purpose ab initio programs have been written using a newly developed procedure for improving the convergence of the reduced linear equation method {P. Piecuch and L. Adamowicz, J. Chem. Phys. 100, 5857 (1994)}. Results of the pilot calculations for few nondegenerate and quasi-degenerate systems are presented and compared with the full configuration interaction data. © 1995 John Wiley & Sons, Inc.
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  • 60
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    International Journal of Quantum Chemistry 55 (1995), S. 173-180 
    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 rigorous approach to the evaluation of the performance of correlated methods in molecular property calculations is proposed. Theoretical descriptions of molecules are identified as collections of molecular property values. Distance functions are then defined in the space of theoretical descriptions and the metric properties are used to define proximity and similarity between theoretical descriptions. Graph theoretic arguments and pattern recognition techniques are used to study ordering, classification, self-organization, and clustering in the space of theoretical descriptions. © 1995 John Wiley & Sons, Inc.
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  • 61
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    International Journal of Quantum Chemistry 56 (1995), S. 175-187 
    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 study a model of the Cooper pair in superconductivity whose basic mechanism is due to nonlinear-nonlocal and non-Hamiltonian interactions; we show the capability of the model of representing available data while possessing intriguing predictive capacities, and we propose its experimental verification. © 1995 John Wiley & Sons, Inc.
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  • 62
    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 all-electron, full-potential LCGTO-FF method, which has been used extensively to calculate the properties of ultrathin films and molecules, is extended to systems with 3-D periodicity in a new computer program called GTOFF. In this investigation, GTOFF is used to calculate the equilibrium properties of hcp beryllium. The structural parameters, binding energy, bulk modulus, and Poisson's ratio obtained with GTOFF are in good agreement with an earlier all-electron, full-potential calculation using the FLAPW method, confirming the high-precision of the present calculations. © 1995 John Wiley & Sons, Inc.
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  • 63
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    International Journal of Quantum Chemistry 56 (1995), S. 217-219 
    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 contribution is concerned with the occurrence of metallic character in K-doped systems and the non-occurrence in Na-doped systems. An N-fold degenerate Hubbard model in the Gutzwiller approximation, previously presented by Jian Ping Lu, is utilized to discuss the metal-insulator transitions observed in these alkali-doped fullerides. The parameters used are derived from INDO calculations in the molecular cluster approach. © 1995 John Wiley & Sons, Inc.
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  • 64
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    International Journal of Quantum Chemistry 56 (1995), S. 235-239 
    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: Calculations on simple systems can be used to study the convergence rates of plane-wave expansions in density-functional methods, to confirm their correctness and numerical stability, and to assess their sensitivity to choice of exchange-correlation functionals and pseudopotentials. This initial communication compares three sets of results on a crystal of noninteracting hydrogen atoms: those obtained from a code we developed for the purpose of making such studies; those generated from the CASTEP code (distributed by Molecular Simulations, Inc.); and those from the PLANE_WAVE code (distributed by Biosym Technologies, Inc.). © 1995 John Wiley & Sons, Inc.
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  • 65
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    International Journal of Quantum Chemistry 56 (1995), S. 241-243 
    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 study the exchange interaction in the quasi-2D antiferromagnetic high Tc superconductor parent compound La2CuO4 by all-electron, embedded cluster methods. Our material model is the cluster Cu2O11, embedded in a matrix of point charges. The SCF ground state configuration is characterized by Cu 2+ and O2- with the d-hole oriented in the [CuO2]2- plane along the Cu—O bonds. We admix to this an excited SCF configuration that has one Cu-hole transferred to the central O into the ground state configuration by nonorthogonal CI, while paying special attention to the variational balance of the singlet and triplet states. This results in a value for the exchange parameter of J = -120 meV, in good agreement with the experimental value of J = -128 meV. © 1995 John Wiley & Sons, Inc.
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  • 66
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    International Journal of Quantum Chemistry 56 (1995), S. 253-256 
    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 article presents some electronic ab initio calculations of ion clusters simulating a piece of alkali halide crystal, both pure and doped with Tl+. In the case of pure crystals, we calculate the (direct) energy band gap from the cluster energy levels. For the case of doped crystals, we calculate the absorption and emission energies and the corresponding Stokes shifts. The cluster model consists of a central cation surrounded by three or four shells of ions, with and without point charges embedding the cluster. © 1995 John Wiley & Sons, Inc.
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  • 67
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    International Journal of Quantum Chemistry 56 (1995), S. 523-531 
    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 dielectric constant of a material is a macroscopic property that measures the reduction of the electrostatic forces between charged plates separated by the material, compared to a vacuum as intermediate material. It is next encountered as a scaling parameter in Coulomb's law for interacting charges, not only in the force, but also in the energy. In deriving the theory for dielectrics, the macroscopic nature is essential: Only then is the basic assumption that the dielectric material is homogeneous and isotropic a valid one. The appearance of the dielectric constant as a simple scaling factor in Coulomb's law has tempted many computational chemists to forget about the macroscopic nature of the dielectric and to apply the screened Coulomb's law between charges, supposedly in a low-dielectric medium such as proteins, in microscopic force fields. Optimization of force fields even led to distance-dependent “dielectric constants.” Another use of the dielectric constant appears in the dielectric continuum reaction field approaches for the computations of solvation energies and solvent effects. The solute is embedded in a cavity surrounded by the dielectric. Specific interactions between solvent molecules and solute are thus neglected. The cavity size and dielectric constants of interior and exterior are optimized for the model. The aim of this article is to show, by means of calculations on interacting point charges embedded in cavities surrounded by dielectrics and microscopic models of “low-dielectric” materials by explicit polarizabilities, that as far as the dielectric “constant” is concerned anything can happen, depending on the nature of the charges, the distance to the cavity boundary, the spatial arrangement of charges, and polarizabilities. Thus, a warning is issued to injudicious use of dielectric models in microscopic calculations. © 1995 John Wiley & Sons, Inc.
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  • 68
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Theoretical and experimental studies were performed on a series of 1,4- and 1,6-dihydronicotinic acid derivatives to investigate their relative stabilities and factors contributing thereto. Ferricyanide-mediated oxidation studies indicated that not only did the 1,4-dihydropyridine derivatives oxidize more slowly than the 1,6-isomers, a finding consistent with previously published literature, but also that the ratio of the rates of reaction was constant. Thus, the 1,4-dihydropyridines oxidized 23-fold more slowly than did the 1,6-derivatives. Theoretical studies also indicated that the 1,4-dihydropyridines were more stable than the 1,6-isomers, as illustrated by lower calculated heats of formation, and were also more resistant to oxidation, as demonstrated by larger HOMO-LUMO energy gaps. An examination of the HOMO for the dihydropyridines showed that the 1,4-isomers were associated with a greater degree of homoaromatic and hyperconjugative stabilization as compared to the 1,6-dihydropyridines and that these factors did not appear to be significantly affected by substitution. These data suggest that the stabilizing properties associated with the 1,4-dihydropyridine system (i.e., greater contributions from the sp3 ring carbon and attached hydrogens) are systematically expressed providing an explanation for the constant difference in experimental reaction rates. © 1995 John Wiley & Sons, Inc.
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  • 69
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    International Journal of Quantum Chemistry 56 (1995), S. 191-199 
    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: Equilibrium structures of intercalation complexes of different metabolites of polycyclic aromatic hydrocarbons - triol carbocations and diol epoxides - with the dG2 · dC2 dinucleotide are obtained by the AMBER and FLEX empirical force field calculations. Triol carbocations derived from benzo[a]pyrene (BaP) and, especially, 7,12-dimethylbenz[a] anthracene (DMBA) are stereochemically compatible with the dinucleotide and B-DNA. The compatibility is caused by two hydrogen bonds between the quasiaxial hydroxyl groups of triol carbocation and the N(3) atoms of successive guanine residues of the DNA fragment, and a van der Waals contact between the C(12) bay region methyl group and methylene group of the deoxyribose residue of the CC strand. This in turn results in better stacking of the reactive centers and, hence, a “preorganization” of the physical complex to subsequent covalent bonding. A substantial out-of-plane deformation of the triol carbocations induced by the bay region methyl group plays an important role in the stereochemical compatibility, and explains the high carcinogenicity of the syn diastereoisomeric form of DMBA metabolites. Specific structural features of the physical complexes “preorganized” for covalent bonding are described. © 1995 John Wiley & Sons, Inc.
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  • 70
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Leukotrienes and thromboxane A 2 are autacoids derived from arachidonic acid (5, 8, 11, 14-icosatretraenoic acid). They are synthesized in cells by 5-lipoxygenase and thromboxane synthase, respectively. Leukotrienes are related to inflammatory and allergic diseases, while thromboxane A2 is a potent platelet aggregating and vasoconstrictor agent involved in cardiovascular pathologies. In this article we have calculated partial potential energy surfaces at the AM1 level for some 5-lipoxygenase inhibitors, thromboxane synthase inhibitors, thromboxane A2 receptor antagonists, and a dual blocker which inhibits thromboxane synthase and antagonizes thromboxane A2 receptor. Our objective was to identify stereoelectronic properties and topographical requirements for these compounds that could be related to their biological activities. Based on our results and on molecular mechanisms of pharmacological action, we were able to propose new potential 5-lipoxygenase inhibitors and dual blockers derived from pyrazole, pyrrole, 1, 2, 3-triazole, and 1, 2, 4-triazole. © 1995 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 56 (1995), S. 1-1 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
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  • 72
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    International Journal of Quantum Chemistry 56 (1995), S. 3-27 
    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
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  • 73
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    International Journal of Quantum Chemistry 56 (1995), S. 41-47 
    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 article described a theoretical approach for treating chemisorption and surface reactions on metals. Electronic structures are described by an ab initio embedding formalism that permits an accurate determination of reaction energetics and adsorbate structure. Chemisorption energies, adsorbate structures, and the reaction of coadsorbed species are reported for methane thiolate on Ni(111) to illustrate the method. © 1995 John Wiley & Sons, Inc.
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  • 74
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    International Journal of Quantum Chemistry 56 (1995), S. 49-59 
    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 accurate description of exchange and correlation effects is a key issue in density functional theory (DFT). In spite of its widespread use, the local density approximation (LDA) has well-known deficiencies. In atoms, the empirical correlation energies are of the same magnitude as the errors of the LDA for exchange. This means that an accurate description of exchange in atoms or molecules is a prerequisite for the introduction of Coulomb correlation. One of the methods that today achieve such an accuracy is the weighted density approximation (WDA). Calculations of the exchange and total energies are presented to substantiate this statement. An optimized local-like approximation (OLA) to Coulomb correlation, based on an explicit modeling of the Coulomb hole, is then combined with the WDA for exchange. The results for total energies and other atomic properties look very promising. A combination of the OLA with several generalized gradient approximations (GGAS) for exchange is explored for molecules. © 1995 John Wiley & Sons, Inc.
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  • 75
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    International Journal of Quantum Chemistry 56 (1995), S. 109-115 
    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 Shannon information entropies of H2, C2, HF, OH, CH, CN, HOOH, C2H6, MeOH, CH3OCH3, CH3F, CH2O, CH3NH2, and C2H4F2 in both position and momentum space were calculated from HF/6-31G* wavefunctions. The results show that there were definite patterns between the information entropies, molecular geometric changes, and energies. The L1, L2, and 1d, measures were compared as effective measures of distance and with energetic changes. The use of these concepts in describing chemical structure is discussed. © 1995 John Wiley & Sons, Inc.
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  • 76
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    International Journal of Quantum Chemistry 56 (1995), S. 593-604 
    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 molecular orbital theory has been used to study the various structures of the SiGeH2 system. Utilizing the DZP basis set in conjunction with five correlated methods, the most intricate being coupled-cluster theory including single, double, and perturbative triple excitations [CCSD(T)], a study of the potential energy surface of the SiGeH2 system was conducted. Equilibrium geometries, harmonic vibrational frequencies, dipole moments, and infrared intensities of the nonplanar dibridged structure, asymmetrical monobridged structures, vinylidenelike structures, trans structure, and linear structure are presented. As anticipated, the dibridged structure was the global minimum, with the novel cis monobridges and a vinylidenelike structure being low lying minima as well, lying within 10 kcal mol-1 of the dibridged global minimum. The two monobridged isomers are predicted to be the third and fourth most stable minima. © 1995 John Wiley & Sons, Inc.
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  • 77
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    International Journal of Quantum Chemistry 56 (1995), S. 609-613 
    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 effects of atomic and molecular electron correlation and Gaunt interaction on the transition energies between the 5D0 and 7F1,2 levels, arising from the 4f6 state of the europium ion, have been studied for the ion and for the [EuO6]9- cluster as it is found in the Ba2GdNbO6 crystal. The calculations were performed using the MOLFDIR program package. The results are compared to work previously done by Visser et al. in which correlation effects had not been taken into account. The effect of the Gaunt interaction is found to be small. An important contribution to the correlation energy emerges from the configuration represented by the double excitations from 4d to 4f. This in agreement with work performed by Jankowski and Sokolowski on Pr3+. The results are now in better agreement with experiment but are still not satisfactory. © 1995 John Wiley & Sons, Inc.
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  • 78
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    International Journal of Quantum Chemistry 56 (1995), S. 627-630 
    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: Variational Monte Carlo techniques are used to compute cross sections for the elastic and inelastic scattering of fast electrons and X-rays by H2. These techniques permit us to use a correlated wave function that would be difficult to handle using conventional techniques. Our results are in good agreement with the best previous calculations. © 1995 John Wiley & Sons, Inc.
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  • 79
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    International Journal of Quantum Chemistry 56 (1995), S. 615-620 
    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 new computer-based theoretical method was developed to study structures and electronic structures of hydrogen-bonded molecules and nanosize clusters, dynamics of proton transfer, and quantum enhancement of reaction rates. While Born-Oppenheimer molecular dynamics (BOMD) simulations allow investigations of the dynamics and thermodynamics of molecules and clusters, this method combines quantum path-integral techniques and density functional theory to treat simultaneously both electrons and hydrogen atoms quantum mechanically. Results from BOMD all-quantum simulations demonstrated the dynamics of proton transfer processes and described the quantum nature of the hydrogen in the protonated systems, respectively. The method also provides a measure of the tunneling enhancement of the “umbrella inversion” in H3O+. © 1995 John Wiley & Sons, Inc.
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  • 80
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    International Journal of Quantum Chemistry 56 (1995), S. 621-625 
    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 molecular dynamics simulation, using a force field established by density functional calculations, was used to investigate structural and vibrational frequency shifts that accompany a compression from 1 bar, 300 K to 143 kbar, 600 K. Under the latter conditions, most pairs of nitromethane molecules were found to be linked by two hydrogen bonds, instead of the one that was observed at 1 bar, 300 K. The computed shifts in the CN and NO, stretching frequencies were in good agreement with the experimental results. © 1995 John Wiley & Sons, Inc.
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  • 81
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    International Journal of Quantum Chemistry 56 (1995), S. 585-592 
    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: Six symmetry-unique local minima of δ-hydroxypentanoic acid, which are stabilized by an intramolecular O ⃛ H—O—C=O hydrogen bond, were located by ab initio self-consistent field (restricted Hartree-Fock) calculations. The geometry data and hydrogen bond parameters of these conformers are presented together with the potential barriers for the various reaction paths. These data are compared to the previous studies on the smaller homologues and the results of investigations on ω-amino acids of the same size. © 1995 John Wiley & Sons, Inc.
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  • 82
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    International Journal of Quantum Chemistry 56 (1995), S. 433-444 
    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 is possible to reformulate the reaction field (RF) model of continuum solvent effects, by considering an approximate expression describing the energy changes from one ground state to another, in the frame of density functional theory (DFT). The energy functional for an arbitrary electronic system coupled to a spin-independent electrostatic external perturbation is used to derive the well-known Born expression giving the electrostatic component of the solvation energy of an atomic ion. The approximate RF-DFT model is illustrated for a series of representative singly positive and negatively charged atomic ions. A Kohn-Sham (KS)-like formalism is then proposed to compute solvation energies within a self-consistent field scheme. The extension of the RF-DFT model to molecular systems is also outlined. © 1995 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 56 (1995), S. 453-476 
    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: Recent reactivity concepts formulated within charge analysis (CSA) are outlined. The charge stability criteria of equilibrium states in open and closed systems are conveniently characterized in terms of the condensed reactant hardness quantities of reactants; their implications for catatytic systems are examined. A use of characteristics associated with selected collective charge displacement modes, including the populational normal modes and minimum-energy coordinates, as diagnostic tools in the theory of chemical reactivity is proposed. The importance of the mapping relations between modes defined in the electron population and nuclear position spaces, respectively, as the unifying concept linking the conjugate charge and geometry displacements, is commented upon. Recent results for model catalytic clusters are used to illustrate some of the concepts introduced. Finally, the relevant contributions to the quadratic interaction energy between reactants are reexamined and expressed in terms of relevant charge sensitivities. © 1995 John Wiley & Sons, Inc.
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  • 84
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    International Journal of Quantum Chemistry 56 (1995), S. 489-498 
    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 Boltzmann-Shannon (BS) information entropy Sρ = ∫ ρ(r)log ρ(r)dr measures the spread or extent of the one-electron density ρ(r), which is the basic variable of the density function theory of the many electron systems. This quantity cannot be analytically computed, not even for simple quantum mechanical systems such as, e.g., the harmonic oscillator (HO) and the hydrogen atom (HA) in arbitrary excited states. Here, we first review (i) the present knowledge and open problems in the analytical determination of the BS entropies for the HO and HA systems in both position and momentum spaces and (ii) the known rigorous lower and upper bounds to the position and momentum BS entropies of many-electron systems in terms of the radial expectation values in the corresponding space. Then, we find general inequalities which relate the BS entropies and various density functionals. Particular cases of these results are rigorous relationships of the BS entropies and some relevant density functionals (e.g., the Thomas-Fermi kinetic energy, the Dirac-Slater exchange energy, the average electron density) for finite many-electron systems. © 1995 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 56 (1995), S. 499-508 
    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 density approximation (LDA) to exchange and correlation effects has well-known limitations. The nonlocal weighted density approximation (WDA) corrects some of those defects. This is illustrated here by applications to free atoms and small atomic clusters. The WDA also induces a nonlocal kinetic energy functional that is tested for atoms. © 1995 John Wiley & Sons, Inc.
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995), S. 277-282 
    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 rational, closed-form Hamiltonian formulation of quantum electrodynamics is presented in a Gaussian basis representation, obtained using a discretized light-cone field quantization method. This approach illustrates the possibility that traditional methods of quantum chemistry may become applicable for the nonperturbative treatment of problems currently accessible only perturbatively if at all, as for example relativistic many-electron interactions in atoms and molecules. © 1995 John Wiley & Sons, Inc.
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  • 87
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    International Journal of Quantum Chemistry 56 (1995), S. 297-301 
    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 approach of Gross, Oliveira, and Kohn for fractionally occupied states of ensembles is applied to determine ensemble exchange potentials for multiplets. A recently proposed method of the author is used to construct exact ensemble exchange potentials of multiplets. © 1995 John Wiley & Sons, Inc.
    Additional Material: 9 Ill.
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  • 88
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995), S. 289-295 
    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 have used several different methods to solve the one-dimensional, time-dependent Schrödinger equation for a sinusiodally modulated barrier. Analytical solutions are given for the case in which the time-dependent part of the potential has several different forms, but is independent of position. For more general fields, Floquet's theorem is used to write the wavefunction as a summation of components which have different energies as a result of the absorption and emission of modulation quanta. A system of coupled ordinary differential equations is obtained, which is then solved numerically using shooting methods. The examples show a resonance in which the tunneling current is markedly increased. For square barriers, this resonance occurs when the particles absorbing modulation quanta are above the barrier, and the length of the barrier is an integer multiple of one-half the de Broglie wavelength. The existence of the resonance is confirmed by asymptotic solutions for large and small frequencies. Examples suggest that it may be possible to make a microwave power amplifier by illuminating a field emitter array with an amplitude-modulated laser operating near the new resonance. © 1995 John Wiley & Sons, Inc.
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  • 89
    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 vibrational frequencies of NaN clusters (2 ≤ N ≤ 72) are calculated by direct diagonalization of the dynamical matrix. Density functional theory with a spherically averaged pseudopotential is used to compute the total energy. The geometry is optimized by the simulated annealing technique. Contributions to the Hessian matrix due to electron relaxation following the ionic displacements are calculated in linear response theory. The frequencies are in the range 0-220 cm-1 and the electron relaxation strongly modifies those of the modes dominated by radial oscillations, particularly the breathing mode frequencies that are proportional to N-1/3. The filling of atomic shells produces a stepwise behavior of the highest frequencies. The giant dipole resonance energies are obtained as a byproduct of the calculation. © 1995 John Wiley & Sons, Inc.
    Additional Material: 9 Ill.
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  • 90
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    International Journal of Quantum Chemistry 56 (1995), S. 657-668 
    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 article presents calculations of the structure, binding energetics, potential energy surfaces, and vibrational spectra of the H5O2+ ion. The 15-dimensional potential energy surface for the seven nuclei in the ionic complex was computed by pointwise ab initio Møller-Plesset second-order perturbation (MP2) calculations, using the correlation-consistent pVTZ basis set augmented with diffuse basis functions on oxygen. The potential energy surface for the proton-transfer mechanism was investigated, and the effects of surrounding water molecules on the proton-transfer potential energy curve was studied. Density functional calculations for the proton-transfer potential surface are compared to the MP2 results. Geometry-optimized structures, binding energies, and harmonic vibrational spectra of H5O2+ and H9O4+ are presented. The energy-minimum structure of H5O2+ using the augmented pVTZ basis set is of C2 symmetry, whereas for H9O4+, using the TZ2P basis set, it is of C3 symmetry. The H-bonded OH stretching harmonic frequency of H5O2+ is very low, 913 cm-1, whereas for H9O4+ it is 2927 cm-1. The subspace spanned by the hydrogen-bonded OH distance and the O—O distance were used in one- and two-dimensional calculations of the anharmonic vibrational spectrum using collocation methods. The coupling of the OH stretch with the O—O vibration causes a redshift and the anharmonicity a blueshift of the OH frequency: the resulting fundamental frequency of the H-bonded OH vibration is 1275 cm-1. Zero-point energies of the proton vibration and pathways for exchange of protons within H5O2+ are discussed. © 1995 John Wiley & Sons, Inc.
    Additional Material: 12 Ill.
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  • 91
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    International Journal of Quantum Chemistry 53 (1995), S. 309-319 
    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 use of effective core potentials in the calculation of the geometrical parameters of the ferrocene molecule and its heavier analogs is reported. It is shown that a critical factor in these calculations is the efective core-core (ECC) potential and, in the absence of ECCs for first-row atoms that are involved in short bonds, calculations of the geometrical parameters are not reliable. Good agreement with experimental geometries may be obtained by using the Los Alamos ECPs for atoms of the second and higher rows of the periodic table at the MP2 level. DFT calculations have been performed and found to give numerical results comparable to MP2 in the same basis. © 1995 John Wiley & Sons, Inc.
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  • 92
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    International Journal of Quantum Chemistry 53 (1995), S. 321-334 
    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 stable structures of substituted polythionylphosphazene single chains have been modeled with small molecular compounds consisting of one repeat unit of the polymer. The geometrical parameters of the nonplanar “trans-cis” conformations of these molecular models are obtained using the ab initio molecular orbital theory. The substituents studies include hydrogen and chlorine atoms and methyl groups. Two basis sets, 3-21G* and 6-31G*, were used in the computations. We have found a very good agreement between the molecular geometries obtained from the two basis sets computations for the methyl-substituted model compounds. The agreement is not as good for the hydrogenated and especially for the chlorinated model compounds. The comparison seems to indicate that the 6-31G* is an essential basis set for the chlorinated compounds. The magnitude of the total dipole moments for these compounds ranges between 3.7 and 7.9 Debye. © 1995 John Wiley & Sons, Inc.
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  • 93
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995) 
    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
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  • 94
    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: Semiempirical molecular orbital techniques (AM1) were applied to the study of cis-trans isomerism in tamoxifen and related compounds. AM1-derived structures were in good agreement with X-ray data. Theoretical evaluation indicated that both cis- and trans-tamoxifen as well as the 4-hydroxytamoxifen isomeric pair were very close in energy, confirming experimental results suggesting nearly equivalent populations of the two isomers under equilibrium conditions. Mechanistic studies included a determination of selected proton affinities to establish the nature and location of compound protonation as well as conformational studies of the formed carbocations. In particular, internal rotation about the incipient olefinic C—C bond in the tamoxifen and 4-hydroxytamoxifen carbocations provided a likely reaction mechanism. There evaluations suggested that the more rapid isomerization of 4-hydroxytamoxifen as compared to tamoxifen was related to transition-state stabilization through phenolic oxygen electron donation. © 1995 John Wiley & Sons, Inc.
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  • 95
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 335-341 
    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: Fluorescence spectroscopy results show that the α-melanocyte-stimulating hormone peptide (α-MSH) interacts with acidic lipid vesicles. Detectable structural changes are concomitant with the passage of a tryptophan residue from aqueous to lipidic media. The observed multiexponential decay of fluorescence, rationalized as originating from three rotameric populations of the tryptophan residue, has been used together with a matrix algorithm to find the most probable conformational families of α-MSH in water and lipid environments. A model is discussed in which the same conformational families occur in various phases, although with different probabilities. A conformational family in which χ1 of the Trp9 side chain is in the trans-rotameric conformation is shown to have structural features highly appropriate to interact with negatively charged biological membranes, which are also in accordance with previous molecular dynamics simulations and with structures engineered in α-MSH analogs that show an increased potency in biological essays. The gauche minus and gauche plus side-chain conformations of Trp9, on the other hand, yield conformations more likely to predominate in aqueous solution. NMR spectroscopy measurements of α-MSH analogs indicate the existence in aqueous solution of a β strand in the vicinity of Trp9. A similar structural feature was found in the present conformational analysis for the gauche minus and gauche plus side-chain rotamers of Trp9. © 1995 John Wiley & Sons, Inc.
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  • 96
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    International Journal of Quantum Chemistry 53 (1995), S. 353-359 
    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 new method is proposed for calculating the ground-state energy and the corresponding electronic correlation energy of molecules with an even number of electrons. A transformation of single-electron terms of the Hamiltonian into terms of the double-electron type and the application of the Hartree-Fock-Bogolyubov variational principle to the new equivalent Hamiltonian are presented. A Bardeen-Cooper-Schrieffer-type ansatz is used for the molecular wave function in analogy with the description of superconductivity in metals. Some test calculations on the ground state of several molecules show the goodness of this method. © 1995 John Wiley & Sons, Inc.
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  • 97
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    International Journal of Quantum Chemistry 53 (1995), S. 607-615 
    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 intermolecular potentials for the X 2σ and A2Π states of Li… Ar were studied by a variety of multiconfiguration, single-configuration, and perturbation methods (CASPT2). The A 2Π excited state was calculated to have a well depth of 811 cm-1 at an internuclear separation of 2.59 Å, in excellent agreement with the 810 cm-1 derived from experimental data. A smaller well of 77 cm-1 was found for the X 2σ ground state at an intermolecular separation of 4.8 Å. These results are in better agreement with experimental results than were the previously reported pseudopotential calculations. The comparison of CI calculation with the CAPST2 results shows that the latter is able to give good results for interacting metal-rare gas systems. © 1995 John Wiley & Sons, Inc.
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  • 98
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    International Journal of Quantum Chemistry 53 (1995), S. 457-466 
    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 perturbation expansion coefficients for the ground-state energy of the half-filled one-dimensional Hubbard model with N = 4ν + 2, (ν = 1, 2,…) sites and satisfying cyclic boundary conditions were calculated in the Hückel limit (U/β → 0), where β designates the one-electron hopping or resonance integral, and U, the two-electron on-site Coulomb integral. This was achieved by solving - order by order - the Lieb-Wu equations, a system of transcendental equations that determines the set of quasi-momenta {ki} and spin variables {τα} for this model. The exact values for these quantities were found for the N = 6 member ring up to the 20th order in terms of the coupling constant B = U/2β, as well as numerically for 10 ≤ N ≤ 50, and the N = 6 Lieb-Wu system was reduced to a system of sextic algebraic equations. These results are compared with those of the infinite system derived analytically by Misurkin and Ovchinnikov [Teor. Mat. Fiz. 11, 127 (1972)]. It is further shown how the results of this article can be used for the interpolation by the root of a polynomial. It is demonstrated that a polynomial of relatively small degree provides a very good approximation for the energy in the intermediate coupling region. © 1995 John Wiley & Sons, Inc.
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  • 99
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 485-493 
    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 energy of weakly overlapping group functions can be written as a series according to the powers of the (σ - I) matrix, where σ is the molecular overlap matrix and I is the unit matrix [1,2]. This power series of the energy is studied by investigating the importance of different order terms to obtain accurate energies and to predict equilibrium bond lengths. It is found that the series is truncated advantageously at an even-order term. Approximate formulas for the first- and second-order terms are proposed in order to reduce computational work. Numerical examples are presented to illustrate the effect of these terms to the energy. The relation of the projection energy to the approximate first- and second-order terms is also discussed. It is found that, by choosing appropriate projection factors, the projection energy corrects the zeroth-order energy more efficiently than does the first-order term. The inclusion of the approximate second-order term represents a slight improvement with respect to the use of the projection energy at the expense of some extra computation. © 1995 John Wiley & Sons, Inc.
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  • 100
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    International Journal of Quantum Chemistry 53 (1995), S. 651-661 
    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 article presents methods for computing Cartesian Gaussian matrix elements using a Taylor series for general potential energy operators that admit well-behaved radial derivatives. These operators arise in the analyses of anharmonic vibrations in molecules. Application to the evaluation of matrix elements for hydrogen associated two wells illustrates the method. © 1995 John Wiley & Sons, Inc.
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