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  • Atomic, Molecular and Optical Physics  (6,266)
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  • 101
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1815-1827 
    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 conformational search was carried out for five opioid peptide homologues and for angiotensin II. Density of states versus energy plots were obtained for each peptide, and the occurrence of common main-chain conformations was investigated by searching homologies between strings of four, five, and six contiguous main-chain amino acid residues rotamers. The results were compared to rates of hydrolysis by endooligopeptidase (EOP) 24.15, known for its specificity for substrate conformations. A catalytic assay of the hydrolysis of angiotensin II was also performed. The two best substrates of EOP 24.15 were found to share unique main-chain conformations and the two worst substrates of EOP 24.15 were found to be nonstructurally homologous to each other and the remaining peptide chains. The conformational search is compared to previous experimental and theoretical results. © 1996 John Wiley & Sons, Inc.
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  • 102
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    International Journal of Quantum Chemistry 60 (1996), S. 1829-1834 
    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 allylic rearrangement of trans-pinocarveol esters (I) to myrtenol esters (II), a reaction of interest in the chemistry of terpenes and cannabinoids, has been theoretically investigated. The intramolecular, cyclization-induced rearrangement results in equilibrium mixtures of the starting compounds and the products with the ratio of I vs. II in the equilibrium mixture being determined by their thermodynamic stabilities. The relative thermodynamic stabilities as reflected by calculated AM1 heats of formations (ΔHf) were determined for various I-II ester pairs. The study, in agreement with available experimental evidence, indicates that generally I, which contain an endocyclic double bond, are more stable and thus predominantly form following rearrangements. In two cases (acetate and pivalate) the stability is reversed. Calculations performed for similar structures, esters of 2-methylene cyclohexane-1-ol (IV) and 1-cyclohexene-1-methanol (V) gave similar results. Structural and electronic factors which might influence the stability of these compounds were examined. Interestingly, a correlation between thermodynamic stability and dipole moments was observed. © 1996 John Wiley & Sons, Inc.
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  • 103
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this work, we evaluated structural and electronic similarities between a new class of acylhydrazones, recently presented as effective inhibitors of a Plasmodium falciparum cysteine protease, and a series of pyrazole arylacylhydrazones with analgesic and antiaggregating (antithrombotic) properties, using AM1. The calculated results suggest that at least one of the pyrazole compounds is similar enough to the active compounds to be considered as a candidate for a future antimalarial series. © 1996 John Wiley & Sons, Inc.
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  • 104
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996) 
    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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  • 105
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. v 
    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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  • 106
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1845-1849 
    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 two- and three-dimensional structure representations with their physicochemical and pharmacological properties; it has also been shown to be a unique conformational descriptor. In the present instance correlation trials were undertaken by regressing reported molecular skin permeability values on their respective calculated three-dimensional FTm indices. The index functioned not only as a structure descriptor but as a natural delineator in the series, separating several outliers in the data set and showing that excellent correlations may be obtained within the classical divisions of chemical structure. These results not only provide a means of estimating skin permeability directly from structure but also may have theoretical implications for studies of the nature and mechanism of dermal transport. © 1996 John Wiley & Sons, Inc.
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  • 107
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    International Journal of Quantum Chemistry 60 (1996), S. 1851-1863 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using the information on the interatomic separations in a molecule, one can construct structural invariants that are the components of a molecular profile. The entries in the profile are derived by averaging different powers of the interatomic separations, suitably normalized so that the increasing powers do not dominate the sequence. Although only a few hundreds of structures have been so analyzed, no two different chemical structures were found to be characterized by the same sequences. A critical test for the conjecture that molecular profiles are unique is to consider structurally closely related systems that are very similar and have several similar properties. In this contribution we investigated the cuboctahedron and the accompanying polyhedron obtained by rotating half of the cuboctahedron against the other half, resulting in the so-called twist cutoctahedron. We show that even this pair of closely related structures has different profiles. We have also examined the generalized molecular profiles obtained by inserting n additional points along each edge of the polyhedra. The convergence of the profiles as n increases is discussed. It appears thus that these generalized molecular profiles, called line profiles or bond profiles, are likely to lead to a unique characterization of structures in which not only the geometry of atoms is recorded but also the geometry of the connectivity of the structure. © 1996 John Wiley & Sons, Inc.
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  • 108
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    International Journal of Quantum Chemistry 60 (1996), S. 1725-1725 
    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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  • 109
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    International Journal of Quantum Chemistry 57 (1996), S. 199-205 
    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 study demonstrates the use of uneven atomic basis sets for ab initio calculations of NMR shielding in the localized orbital/local origin (LORG) approach with norbornenone as the test case. We distinguish between locally dense sets (extended basis on target atom only) and locally saturated sets (extended bases on target atom and atoms in its first bonding sphere), using 6-311G** and 6-31G sets to describe the high and low level of function sets. It is shown that the use of these uneven sets can simulate high basis set level calculations of shieldings for 1H and for all the 13C nuclei in this molecule and, hence, allows quite accurate ab initio calculations of shielding properties of these nuclei in large molecules using relatively modest computational facilities. The shielding of the double-bonded 17O nucleus is apparently sensitive to basis-set quality beyond the first bonding sphere. © 1996 John Wiley & Sons, Inc.
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  • 110
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    International Journal of Quantum Chemistry 57 (1996), S. 207-212 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bond function basis sets combined with the counterpoise procedure are used to calculate the molecular dissociation energies De of 24 diatomic molecules and ions. The calculated values of De are compared to those without bond functions and/or counterpoise corrections. The equilibrium bond lengths re and harmonic frequencies οe are also calculated for a few selected molecules. The calculations at the fourth-order Møller-Plesset approximation (MP4) have consistently recovered about 95-99% of the experimental values for De; compared to as low as 75% without use of bond functions. The calculated values of re are typically 0.01 Å larger than the experimental values, and the calculated values of οe are over 95% of the experimental values. © 1996 John Wiley & Sons, Inc.
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  • 111
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    International Journal of Quantum Chemistry 57 (1996), S. 235-243 
    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: The diagonalization of a class of lattice spin models of a particular structure is first reviewed and secular polynomials for these models are calculated explicitly from the corresponding secular matrices. The structure of the eigenvectors of the given secular matrices is investigated and used to determine the eigenvalues theoretically, and proofs which have not appeared are presented. These results can be compared to the results obtained from the full secular polynomials. © 1996 John Wiley & Sons, Inc.
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  • 112
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    International Journal of Quantum Chemistry 57 (1996), S. 501-518 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A recently proposed pair population scheme is investigated for ab initio SCF and spin-coupled wave functions. All the required expressions are rederived, in very compact form, and efficient computational schemes are presented for their evaluation. As representative examples of different types of bonding, pair populations and related quantities are examined for H2, LiH, BeH, CH4, H2O, and B2H6. Various “effective valencies” are also considered. © 1996 John Wiley & Sons, Inc.
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  • 113
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    International Journal of Quantum Chemistry 57 (1996) 
    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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  • 114
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    International Journal of Quantum Chemistry 57 (1996), S. 567-573 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A study of the coronene - coronene association using different interaction potentials based on an atom-atom pair potential proposed by Fraga has been performed. The interaction potentials employed differ in the way the electrostatic and/or dispersion contributions are computed. The influence of both contributions on the geometries predicted for the coronene dimer is discussed in order to analyze the effectiveness of the different interaction potentials. The stationary points found in each interaction energy hypersurface are characterized by calculating the Hessian eigenvalues. Results are discussed in the light of those previously reported for the benzene dimer. Stacked-displaced structures are suggested to be the preferred conformations for the coronene - coronene association. © 1996 John Wiley & Sons, Inc.
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  • 115
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    International Journal of Quantum Chemistry 57 (1996), S. 575-585 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spectroscopic properties of all the electronic states of KRb dissociating into 4s(K) + 5s(Rb), 4s(K) + 5p(Rb), 4p(K) + 5s(Rb), and 4s(K) + 4d(Rb) and some higher-lying excited states are studied with ab initio calculations. Spectroscopic constants, dipole moments, and the nature of the electronic wave functions for these states are reported. Intensities for the singlet-singlet and triplet-triplet transitions are theoretically calculated from the potential energy curves and the transition dipole moments. © 1996 John Wiley & Sons, Inc.
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  • 116
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    International Journal of Quantum Chemistry 57 (1996), S. 587-594 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The outer-valence ionization potentials, low-lying electronically excited states, and 1J(CC) nuclear spin-spin coupling constants of bicyclobutane, tricyclopentane, tricyclohexane, and octabisvalene have been investigated at the ab initio level by using the outer-valence Green function (OVGF), random-phase approximation (RPA), and equations-of-motions (EOM) approaches, which incorporate the main dynamical correlation effects. A comprehensive theoretical prediction of these spectroscopic observables is presented, which shows several interesting aspects stemming from the unique electronic structure of the bridge bond. © 1996 John Wiley & Sons, Inc.
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  • 117
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    International Journal of Quantum Chemistry 57 (1996), S. 611-623 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A full theoretical treatment of electron capture processes using ab initio configuration interaction methods within, according to the collision energy range concerned, a semiclassical or a quantal collisional formalism including translation effects has been developed recently. An application for collisions involving boron, an important impurity in fusion reactors, is presented on examples of the ground state: B3+(1s2) + He, B4+(1s) + H, and the metastable ion B3+(1s2s) + H reactions. © 1996 John Wiley & Sons, Inc.
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  • 118
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    International Journal of Quantum Chemistry 57 (1996), S. 7-15 
    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 solution of the Schrödinger equation for the d-dimensional hydrogen atom in a d-dependent potential defined by Gauss' law has been studied by the shifted 1/d method and the δ expansion. These methods provide analytical formulas for the eigenvalues and eigenfunctions which have been tested against “exact” numerical values obtained recently. The comparison shows that the results obtained with the analytical expressions are in excellent accord with the numerical ones and, on the other hand, provide a theoretical justification for a proposed empirical expression for the energies of the two-dimensional hydrogen atom with a logarithmic potential energy function. © 1996 John Wiley & Sons, Inc.
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  • 119
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    International Journal of Quantum Chemistry 57 (1996), S. 43-51 
    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: Analytical exact expressions are obtained for matrix elements of the modified Pöschl-Teller oscillator over different operators including powers of the hyperbolic functions sinh(α x), cosh(α x), and tanh(α x) and the differential operators d/dx and d2/dx2. These expressions are derived using explicitly the Pöschl-Teller eigenfunctions. In addition, several recursion relations connecting different Pöschl-Teller matrix elements are obtained using the factorization and hypervirial techniques. It is shown that these relations can be used to make easier the computation of the matrix elements. © 1996 John Wiley & Sons, Inc.
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  • 120
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    International Journal of Quantum Chemistry 57 (1996), S. 105-111 
    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 chemisorption of atomic hydrogen and oxygen on a cobalt surface has been studied on a five-atom cluster model using one-electron effective core potential (1e-ECP) and all-electron calculations at the ab initio SCF and MCPF levels. Also, density functional calculations have been carried out. The different approaches are evaluated. The 1e-ECP has been compared to similar ECPS for nickel and copper. Our results indicate that this approach is valid also for cobalt. Different contributions to the cluster-adsorbate bonding energy are discussed. © 1996 John Wiley & Sons, Inc.
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  • 121
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    International Journal of Quantum Chemistry 57 (1996), 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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  • 122
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    International Journal of Quantum Chemistry 58 (1996) 
    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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  • 123
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    International Journal of Quantum Chemistry 58 (1996), S. 539-540 
    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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  • 124
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    International Journal of Quantum Chemistry 58 (1996), S. 537-537 
    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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  • 125
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    International Journal of Quantum Chemistry 59 (1996), S. 3-6 
    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: Irregularities in the connection of the high- and low-density forms of the Perdew-Zunger correlation energy functional lead to computational difficulties that are removed by a new parametrization. These parametrizations and a number of other correlation functionals are compared and discussed. © 1996 John Wiley & Sons, Inc.
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  • 126
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    International Journal of Quantum Chemistry 59 (1996), S. 15-29 
    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 general expression for the sixth-order Møller-Plesset (MP6) energy, E(MP6), has been dissected in the principal part A and the renormalization part R. Since R contains unlinked diagram contributions, which are canceled by corresponding terms of the principal part A, E(MP6) has been derived solely from the linked diagram terms of the principal part A. These have been identified by a simple procedure that starts by separating A into connected and disconnected cluster operator diagrams and adding terms associated with the former fully to the correlation energy. After closing all open disconnected cluster operator diagrams, one can again distinguish between connected and disconnected energy diagrams, of which only the former lead to linked diagram representations and, therefore, contributions to E(MP6). The connected diagram parts of A have been collected in four energy terms E(MP6)1, E(MP6)2, E(MP6)3, and E(MP6)4. The sum of these terms has led to an appropriate energy formula for E(MP6) in terms of first- and second-order cluster operators. © 1996 John Wiley & Sons, Inc.
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  • 127
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    International Journal of Quantum Chemistry 59 (1996), S. 71-95 
    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
    Notes: Three different ways of getting reliable estimates of full configuration interaction (FCI) correlation energies are tested, namely (a) by Padé approximants [k, k] and [k, k - 1], (b) by using extrapolation formulas, and (c) by Feenberg scaling of Møller-Plesset (MP) correlation energies. By using MPn energies up to sixth order, i.e., MP2, MP3, MP4, MP5, and MP6, it was possible to test the convergence behavior of the Padé series [1, 0], [1, 1], [2, 1], [2, 2] and the Feenberg series up to sixth order where in the latter case a scaling factor λ(5) (scaling of the second-order wave function, FE2) rather than the previously tested λ(3) (scaling of the first-order wave function, FE1) was considered. Investigation of 26 different correlation energies for systems with monotonic convergence in the MPn series (class A systems) or initially oscillatory convergence behavior (class B systems) indicates that Padé approximants lead in some cases to reasonable estimates of FCI correlation energies, but in other cases, in particular for class B systems, they give too negative correlation energies. Both monotonic and oscillatory behavior for the Padé series is observed where it is possible to predict its convergence behavior on the basis of calculated MPn energies. The best estimates of the FCI correlation energy are obtained by FE2 scaling. At sixth-order FE2, values for atoms and molecules with equilibrium geometry differ on the average by just 0.146 mhartree from FCI correlation energies. The FE2 correlation energies all converge monotonicly. Also, FE2 scaling reduces the exaggeration of MP6 correlation energies for class B systems. However, surprisingly good estimates of FCI energies are also obtained by simple extrapolation formulas based on MP4, MP5, and MP6 correlation energies. © 1996 John Wiley & Sons, Inc.
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  • 128
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    International Journal of Quantum Chemistry 59 (1996), S. 97-102 
    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 characteristic polynomial associated with π-electrons of conjugated molecules are discussed by using subgraphs derived from molecular graphs as a basis for their construction. A practical method has been developed for evaluating the coefficient aK of conjugated molecules. Applying this method, the general formulas of evaluating the coefficient aK for homologous conjugated molecules have been obtained. The approach is illustrated on a few simple conjugated systems, including also a few polymeric systems. © 1996 John Wiley & Sons, Inc.
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  • 129
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    International Journal of Quantum Chemistry 59 (1996), S. 103-108 
    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 are reported of X3zz,zz,z,z(-iω;iω,0,0), X3zz,zz,x,x(-iω;iω,0,0), X3zzz,z,z,z(-iω;iω,0,0), X4zz,z,z,z,z(-iω;iω,0,0,0), and X4zz,x,x,z,z(-iω;iω,0,0,0) for H and He. These are the independent components of the atomic (quadrupole)2-(dipole)2, (octupole)-(dipole)3, and (quadrupole)-(dipole)4 hyperpolarizability tensors for an imaginary frequency iω. The first three quantities have been used for the determination of the R-8 dispersion contribution to the H-H, H-He, and He-He collision-induced pair polarizability functions. The last two quantities will be needed in future calculations of the leading order (R-7) dispersion contributions to the collision-induced first hyperpolarizability (β) for the H-He pair. The calculations were based on sum-over-states formulations with near-exact wave functions for H and highly accurate explicitly electron-correlated wave functions for He. For this reason we believe them to be of benchmark quality. © 1996 John Wiley & Sons, Inc.
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  • 130
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    International Journal of Quantum Chemistry 59 (1996), S. 109-117 
    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 local grid method proposed earlier is used to model chemical dynamical events in more than one dimension. Two different mean-field routes are applied to model problems representing dynamics of isomerization, H+-ion transfer, energy transfer, etc. The methods are seen to work with equal facility for both time-dependent and time-independent potentials. © 1996 John Wiley & Sons, Inc.
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  • 131
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    International Journal of Quantum Chemistry 57 (1996) 
    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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  • 132
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    International Journal of Quantum Chemistry 57 (1996), S. 811-821 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio dynamic polarizabilities per unit cell of infinite stereoregular molecular hydrogen chains are calculated at the coupled Hartree-Fock level of approximation by using the random-phase approximation and the STO-3G and double-zeta atomic basis sets. Comparison with molecular calculations on increasingly large oligomeric chains emphasizes the nice extrapolation property of the polymeric technique that provides asymptotic values very close to the largest oligomeric values. The poles of the polarization propagator associated with the electric dipole polarizability correspond to the singlet excitation energies. Comparisons are performed with other techniques that provide the band gap. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 57 (1996), S. 823-841 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of detailed theoretical investigations of the properties of atomic and diatomic H in GaAs were analyzed with the effort to give a unified picture of the H behavior in this semiconductor. All calculations were performed in the pseudopotential density-functional framework using a supercell approach. We studied both shallow impurities (Si and C) and deep point defects (As antisite and Ga vacancy). Generally, a simple scheme may be applied in order to describe the H interaction with shallow impurities, where a key role is played by the amphoteric character of H. More complex mechanisms are involved in the deep impurity case that are related to new, interesting effects of H incorporation in GaAs. © 1996 John Wiley & Sons, Inc.
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  • 134
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    International Journal of Quantum Chemistry 57 (1996), S. 843-849 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electrostatic potential and the intensity of the electric field above the isolated layer of the phyllosilicate mineral talc (layer group symmetry C2/m) were computed using the semiempirical INDO/2 method. The electrostatic potential Vc and the intensity of electric field (OVERBAR)Ec, above the surface of semi-infinite crystal were obtained as the sum of the contributions of the infinite number of individual layers. The interaction energy Uint between a noble gas atom and the talc crystal was computed as (a) a pure Lennard-Jones energy ULJ and (b) the sum of ULJ and interaction energy with induced dipole moment: Uind = α|(OVERBAR)E|2, where α is the dipole polarizability of the noble gas atom. The one-particle configurational integral was calculated within the classical mechanics limit. Both the free and adsorbed gas phases were assumed to be ideal. The obtained results demonstrate that in the case of uncharged and nonpolar adsorbate and an uncharged surface the electrostatic part of the interaction energy should be included in the total interaction energy. © 1996 John Wiley & Sons, Inc.
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  • 135
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    International Journal of Quantum Chemistry 57 (1996), S. 851-860 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio crystal orbital calculations on three-dimensional crystals of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), diprotonated deoxycytidine-5′-monophosphate monohydrate (5′-dCMP*2H*1Water), disodium deoxyguanosine-5′-monophosphate tetrahydrate (5′-dGMP*2Na*4Water), disodium uridine-3′-monophosphate tetrahydrate (3′-UMP*2Na*4Water), monosodium monoprotonated deoxyadenosine-5′-monophosphate hexahydrate (5′-dAMP*1H*1Na*6Water), disodium deoxycytidine-5′-monophosphate heptahydrate (5′-dCMP*2Na*7Water), cis-polyacetylene (cis-PA), and polythiophene (PTP) were carried out using the CRYSTAL92 routine package. A suitable basis set has been found that enables one to carry out the above calculations at the STO-3G level of accuracy using SILICON GRAPHICS workstations. However, to obtain reasonable results for three-dimensional crystals of polymers, one has to use much more extended basis sets. © 1996 John Wiley & Sons, Inc.
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  • 136
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    International Journal of Quantum Chemistry 59 (1996), S. 147-165 
    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 problem of excitons in interaction with phonons in a molecular crystal has been reinvestigated as a continuation of our earlier work. The exciton-phonon interaction has been taken to be linear in lattice displacements. The external medium, the phonon assembly, has been considered to be in thermal equilibrium. Following Simons, we have incorporated the effects of the medium on the exciton dynamics into a time-dependent effective potential that contains the equilibrium average exciton-phonon interaction as well as terms arising from the fluctuations in the medium's coordinates about their equilibrium values. A correlation function that represents the probability of exciton transfer has been given in the interaction picture. The time evolution of this correlation function has been determined by following Kubo's technique of cumulant expansion. The zeroth-, second-, and fourth-order contributions to the correlation function have been calculated in this way. The second- and fourth-order contributions have been diagrammatically represented. The second-order contribution has been explicitly calculated in different physical limits, namely, the slow exciton and the slow phonon limits at high and low temperatures and for very large and very small time. A few simple formulas for the transfer probability of a bare exciton in a molecular crystal of cubic symmetry have been derived from the Debye approximation for the dispersion of phonons. It has been specifically shown that the sum over phonon modes in the large time dynamics leads to a fully destructive interference in second order at a very low temperature and gives rise to a diffusive transport at a high enough temperature. A natural way of clothing the excitons has been considered and the clothed exciton has been represented diagrammatically. The dressing requires the correlation function to be redefined in terms of the clothed states and the clothed operators. The clothed exciton correlation function that represents the probability of transfer of excitons fully clothed by the phonons in thermal equilibrium turns out to be identical with the bare exciton correlation function. This attaches a novel interpretation to the correlation function which was originally defined by Simons. Transfer probabilities for a clothed exciton in a cubic crystal has been explicitly worked out for different physical limits under the Debye model of phonon dispersion. From these results a few expressions for the macroscopic diffusion coefficient of the clothed exciton have been obtained. A few critical comments have been incorporated. © 1996 John Wiley & Sons, Inc.
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  • 137
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    International Journal of Quantum Chemistry 59 (1996), S. 173-182 
    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 simple formalism of real irreducible tensorial sets of real bases is proposed. The definition of the real bases, the coupling of the real bases, and the transformation of the real bases in a group chain including the three-dimensional rotation group and the molecular point groups are studied. The double coset technique is used to derive the close formulas for generating the coupling coefficients and the transformation coefficients. © 1996 John Wiley & Sons, Inc.
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  • 138
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    International Journal of Quantum Chemistry 57 (1996), S. 141-155 
    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: The matrix differential calculus is applied for the first time to a quantum chemical problem via new matrix derivations of integral formulas and gradients for Hamiltonian matrix elements in a basis of correlated Gaussian functions. Requisite mathematical background material on Kronecker products, Hadamard products, the vec and vech operators, linear structures, and matrix differential calculus is presented. New matrix forms for the kinetic and potential energy operators are presented. Integrals for overlap, kinetic energy, and potential energy matrix elements are derived in matrix form using matrix calculus. The gradient of the energy functional with respect to the correlated Gaussian exponent matrices is derived. Burdensome summation notation is entirely replaced with a compact matrix notation that is both theoretically and computationally insightful. © 1996 John Wiley & Sons, Inc.
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  • 139
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    International Journal of Quantum Chemistry 57 (1996), S. 183-198 
    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
    Notes: The virial field V(r) is defined by the local statement of the quantum mechanical virial theorem, as the trace of the Schrödinger stress tensor. This field defines the electronic potential energy density of an electron at r and integrates to minus twice the electronic kinetic energy. It is the most short-ranged description possible of the local electronic potential energy and it exhibits the same transferable behavior over bounded regions of real space (corresponding to the functional groups of chemistry) as does ρ(r). This article establishes a structural homeomorphism between -V(r) and ρ(r), showing that the two fields are homeomorphic over all of the nuclear configuration space. The stable or unstable structure defined by the gradient vector field Δρ(r; X) for any configuration X of the nuclei can be placed in a one-to-one correspondence with a structure defined by the field -ΔV(r; X′). In particular, a molecular graph for ρ(r) defining a molecular structure is mirrored by a corresponding virial graph for V(r) and the lines of maximum density linking bonded nuclei in the former field are matched by a set of lines of maximally negative potential energy density in the latter. The homeomorphism is also geometrically faithful, an equilibrium geometry in general, exhibiting equivalent structures in the two fields. The demonstration that the virial field, whose integrated value equals twice the total energy, is essentially just a locally scaled version of the electron density is suggestive of possible new approaches in density functional theory. © 1996 John Wiley & Sons, Inc.
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  • 140
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    International Journal of Quantum Chemistry 57 (1996), S. 219-227 
    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 first hydrogen bond of the guanine-cytosine (G-C) pair, the potential function V(r, R) is found for the nonequilibrium bond length R. A new form of the semiempirical potential function, Clementi's calculations for V(r, R0) and some experimental data are used. New wave functions and energy levels as a function of the H-bond length for the ground and several excited states are obtained. This allows one to obtain frequencies and amplitudes of some vibrations of the G-C pair side groups involved in the formation of the hydrogen bond when the latter is not excited and when it is excited to the first energy level. © 1996 John Wiley & Sons, Inc.
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  • 141
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    Notes: The influence of a hydroxyl group simulating Ser-48 in the hydride-transfer step characteristic of liver alcohol dehydrogenase is studied on the hydride-transfer reaction as modeled by a methanolate anion interacting with a cyclo propenyl cation. It is shown first that this is an adequate model by comparing it to the methanolate-pyrydinium cation model transition structure, (TS). The side-chain effect is modeled first by adding water and then with methanol located at the position that Ser-48 occupies in the enzyme; a supermolecule approach is used. It is found that (i) the normalized advance coordinate (NAC) for the exchanged hydrogen has an invariant value at the TS and the reactant, while for the product, the NAC depends upon the external perturbation introduced by the ancillary molecule (the TS is reactant-like); (ii) the products are strongly destabilized, so the (activation) barrier with respect to the TS diminishes; (iii) the energy gap between reactants and products is sensibly diminished by the presence of methanol; (iv) the alcoholate moiety in the hydride transfer complex is not spontaneously protonated; and (v) there is a negligible charge transfer between the hydride-transfer system and models of Ser-48. In the present simplified model, methanol appears to have a catalytic effect via hydrogen bonding. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 57 (1996), S. 261-261 
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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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  • 143
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    International Journal of Quantum Chemistry 57 (1996), S. 281-293 
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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: It is shown how the regularized two-component relativistic Hamiltonians of Heully et al. and Chang, Pelissier, and Durand can be viewed as arising from a perturbation expansion that unlike the Pauli expansion remains regular even for singular attractive Coulomb potentials. The performance of these approximate Hamiltonians is tested in the framework of the local density approximation and the relation of their spectrum to that of the Dirac Hamiltonian is discussed. The circumstances under which the current approximations are superior to the Pauli Hamiltonian are analyzed. Finally, it shown how the Hamiltonians could be used within the context of conventional Hartree-Fock theory. © 1996 John Wiley & Sons, Inc.
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  • 144
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    International Journal of Quantum Chemistry 57 (1996), S. 321-325 
    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 sampling of the exact solution of the Schrödinger equation by quantum Monte Carlo methods allows one to solve the problem of the optimization of linear and nonlinear parameters of a trial wave function by minimization of the distance to the exact wave function in Hilbert space even for those systems whose exact wave function is unknown. The overlap integrals between the basis functions and the exact wave function can be easily estimated within the quantum Monte Carlo formalism. Several observables of the helium atom ground state, computed both within the orbital approximation and by an explicitly correlated basis set, evidence the overall goodness of the wave function optimized according to this criterion. © 1996 John Wiley & Sons, Inc.
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  • 145
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    International Journal of Quantum Chemistry 57 (1996), S. 871-879 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio RHF/STO-3G, 3-21G, and 6-31G band structure calculations were carried out on an extended regular chain of beryllium atoms ((SINGLEBOND)Be2(SINGLEBOND))∞ to study its stability in comparison with corresponding data on finite chains and previously reported results on small elemental beryllium clusters. It is found that, starting from a linear distribution of atoms along the chain, the system evolves toward a regular zigzag planar structure. The total RHF energy per constituent atom is higher for the chains than for the clusters and is thereby suggestive of that isolated chains are unlikely to exist except in constraining channels of host structures. In the limit of the infinite chain, there are signs indicating that the RHF model is no longer an adequate description. © 1996 John Wiley & Sons, Inc.
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  • 146
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    International Journal of Quantum Chemistry 57 (1996), S. 881-886 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The assumption of the concentration dependence of the energy of formation of vacancy-type defects (VTD) in solids is applied to the procedure of the estimation of the defect interaction parameters. The VTD effective interaction is described by the mixing potential. The sign of this potential characterizes the stability of the VTD-atom “alloy.” Employing the example of Nb, we illustrate the ability of the suggested approach to describe the stability of the VTD-atom “alloy.” © 1996 John Wiley & Sons, Inc.
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  • 147
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    International Journal of Quantum Chemistry 57 (1996), S. 887-896 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Extensive self-consistent real-space recursion-method calculations were performed for the Pt overlayer or the Pt7 cluster on the Ni(111) surface and for hydrogen chemisorption on these systems. Correlations between the surface-atom local density of electronic-state properties before chemisorption, surface reactivity, and the initial-state contribution to the metal core-level shifts, respectively, is documented and discussed. The experimentally observed catalytic properties of Pt—Ni alloy surfaces are also briefly considered. © 1996 John Wiley & Sons, Inc.
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  • 148
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    International Journal of Quantum Chemistry 57 (1996), S. 897-901 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the basis of a nonempirical pair potential, the temperature dependencies of the long-range-order parameter of NiTi and AuPd are calculated. The suggested effective potential approach allows one to describe the changes in phase relations. The evaluation of the internal mixing energy of alloys is carried out in the framework of the static concentration waves theory of Khachaturyan. The model outlines the technique to calculate the values of the energy parameters of the ordering process from first principles. © 1996 John Wiley & Sons, Inc.
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  • 149
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    International Journal of Quantum Chemistry 57 (1996), S. 259-259 
    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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  • 150
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    International Journal of Quantum Chemistry 57 (1996), S. 265-280 
    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: Sequence transformations, which transform a slowly convergent or divergent sequence {sn}∞n&=0 into a new sequence {sln}∞n=0 with hopefully better numerical properties, are useful computational tools to overcome convergence and divergence problems. In this article, sequence transformations are discussed that use explicit remainder estimates [E.J. Weniger, Comput. Phys. Rep. 10, 189 (1989)]. Because of the explicit incorporation of the information contained in the remainder estimates, these transformations are potentially very powerful and well suited for the summation of strongly divergent series. The Rayleigh-Schrödinger perturbation series for the ground-state energy of the quartic, sextic, and octic anharmonic oscillator is a typical example of a perturbation series that diverges quite strongly for every nonzero coupling constant. It can be summed efficiently even in the very challenging strong coupling regime, if sequence transformations are combined with a suitable renormalization technique described by F. Vinette and J. C˘íz˘ek [J. Math. Phys. 32, 3392 (1991)]. Moreover, a renormalized strong coupling expansion for the ground-state energy of an anharmonic oscillator can be constructed, which apparently converges for all coupling constants β ε [0,∞) and which makes the computation the ground-state energy almost trivial. Other applications of sequence transformations in quantum mechanical and quantum chemical calculations are also reviewed. © 1996 John Wiley & Sons, Inc.
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  • 151
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    International Journal of Quantum Chemistry 57 (1996), 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: Gradient corrections to the local spin density (LSD) approximation for the exchange-correlation energy are making density functional theory as useful in quantum chemistry as it is in solid-state physics. But which of the many gradient-corrected density functionals should be preferred a priori? We make a graphical comparison of the gradient dependencies of some popular approximations, discussing the exact formal conditions which each obeys and identifying which conditions seem most important. For the exchange energy, there is little formal or practical reason to choose among the Perdew-Wang 86, Becke 88, or Perdew-Wang 91 functionals. But, for the correlation energy, the best formal properties are displayed by the nonempirical PW91 correlation functional. Furthermore, the real-space foundation of PW91 yields an insight into the character of the gradient expansion which suggests that PW91 should work especially well for solids. Indeed, while improving dissociation energies over LSD, PW91 remains the most “local” of the gradient-corrected exchange-correlation functionals and, thus, the least likely to overcorrect the subtle errors of LSD for solids. To show that our analysis of spin-unpolarized functionals is sufficient, we also compute spin-polarization energies for atoms, finding PW91 values only slightly more negative than LSD values. © 1996 John Wiley & Sons, Inc.
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  • 152
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    International Journal of Quantum Chemistry 57 (1996), S. 453-463 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An approach for a space-separated calculation of the wave function in the valence and core regions of a molecule is proposed. As the first step, the calculation of the orbitals (or two-component spinors in the relativistic case) in the valence region by the effective core potential (ECP) method is performed. Then, it is followed by a restoration of orbitals (four-component spinors) expanded on spherical harmonics in the core regions of heavy atoms. Theoretical questions of the variational calculation of the molecular orbitals are considered in some core region limited by a sphere. Inclusion from the electronic cloud outside this region is reduced by the necessity of taking into account the orthonormality and boundary conditions together with an effective external field in respect to the selected core region.This method may be used for calculation of matrix elements of operators that are singular near nuclei (P, T-odd interactions, hyperfine structure, etc.). A substantial computational saving can be reached because the method enables, by the most optimal way, to combine the advantages of two well-developed approaches: molecular ECP calculations in the Gaussian basis set and one-center numerical atomic calculations with an external field. It is especially important when the relativistic effects are taken into account. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 57 (1996), S. 471-479 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Four-component relativistic and nonrelativistic molecular orbital calculations were performed for the covalent paramagnetic complex [Ir(CN)5]3-, employing the self-consistent discrete variational method, in the framework of density functional theory. Relativistic effects on the electronic structure and chemical bonding are discussed by comparison of relativistic and nonrelativistic one-electron energy levels, populations, and bond orders. The influence of relativistic effects on calculated absorption energies of the electronic spectrum is briefly assessed. © 1996 John Wiley & Sons, Inc.
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  • 154
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    International Journal of Quantum Chemistry 57 (1996), S. 481-491 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Insertion of CO2 into the transition metal-hydride bond of [RhIIIH2(PH3)3]+, CuIH(PH3)2, and RhIH(PH3)3 was theoretically investigated with ab initio MO/MP4, SD-CI, and CCD methods. The geometries of reactants, transition states (TS), and products were optimized at the Hartree-Fock level, and then MP4, SD-CI, and CCD calculations were performed on those optimized structures. The TS of the CO2 insertion into the CuI(bond)H bond is the most reactantlike, while the TS of the CO2 insertion into the RhIII(bond)H bond is the most productlike. The activation energy (Ea) and the reaction energy (ΔE) were calculated to be 6.5 and -33.5 kcal/mol for the CO2 insertion into the Cu1(bond)H bond, 21.2 and -7.0 kcal/mol for the CO2 insertion into the Rh1(bond)H bond, and 51.3 and -1.1 kcal/mol for the Rh111(bond)H bond at the SD-CI level, where negative ΔE represents exothermicity. These results are discussed in terms of the M(bond)H bond energy and the trans-influence of the hydride ligand. © 1996 John Wiley & Sons, Inc.
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  • 155
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    International Journal of Quantum Chemistry 57 (1996), S. 519-532 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the grounds of results from quantum chemical calculations, we introduce spin-catalysis as a new concept, and classify different types of chemical reactions according to this concept. We propose a general definition of spin-catalysis as phenomena in which chemical reactions are promoted by substances which assist in overcoming spin-prohibition or in which the activation barrier is lowered through spin uncoupling induced by a paramagnetic catalyst. A number of known phenomena fall into this definition and can be classified according to two main categories: (1) spin-orbit coupling induced and (2) paramagnetic-exchange induced spin-catalysis. Other types of spin-catalysis are also discussed: (3) processes with participation of substances which assist in (a) photochemical and (b) thermal or electrochemical generation of active particles by energy and - by electron transfer [radicals, diradicals, O2(1Δg), etc.], reacting further without spin-prohibition: (4) processes induced by an external magnetic field. Processes (3) and (4) are quite general and well known; their inclusion in the spin-catalysis classification does not introduce any new findings for the chemical kinetics, but the general features, which unite them with the important catalytic processes of the first two types, serve as a useful guide in catalysis theory. Few models of spin-catalytic processes have been simulated for the purpose of illuminating the principles of spin-catalysis; the cis-trans isomerization of ethylene catalyzed by molecular oxygen, the external heavy atom effect in ethylene photochemistry, and some others. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 57 (1996), S. 533-542 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The performance of numerical basis sets in relation to Gaussian basis sets is examined, by studying 20 small sulfur-containing molecules. The results of geometry optimization calculations are reported for each molecule using both density functional and Hartee-Fock methods. In comparison with experimental data, it is shown that the use of numerical bases tend to overestimate structural parameters, particularly bond lengths, and, in most cases, more than Gaussian basis sets. It is also shown that the use of a larger Gaussian basis set in DFT calculations has the effect of reducing bond lengths. © 1996 John Wiley & Sons, Inc.
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  • 157
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    International Journal of Quantum Chemistry 57 (1996), S. 663-671 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Many of the important properties of transition-metal complexes depend on the low-energy excitation spectrum formed by d-electrons of the central transition-metal atom. The spectra of this type are usually fit to the well-known crystal field theory or to the angular overlap model. The result of the fitting is a set of parameters which are considered as characteristics of the electronic structure of the complex such as strength of the ligand field or types and extent of metal-ligand bonding. We present here a short account of the effective Hamiltonian method recently developed to calculate the splitting of the d-levels by the ligands and the resulting d-d spectra of transition-metal complexes together with some results of its application to the mixed-ligand complexes with the general formula ML4Z2, where M = V, Co, Ni; L = H2O, NH3, Py; and Z = H2O, NCS-,C -l. Particular attention is paid to the V(H2O)4Cl2 and Co(H2O)4Cl2 compounds. The former seems to have tetragonal structure, whereas for the latter, our method predicts a spatially degenerate ground state for the tetragonal arrangement of the ligands. That must lead to the Jahn-Teller distortion, which is actually observed. © 1996 John Wiley & Sons, Inc.
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  • 158
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    International Journal of Quantum Chemistry 57 (1996), S. 697-705 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of ab initio RHF/3-21G, RHF/6-31G*, and MP2/6-31G** / / HF/6-31G* calculations for 10 possible configurations of OM4H6 molecules (MO · 3MH2, M = Be, Mg) are reported. Five isomers of OBe4H6 and three isomers of OMg4H6 have been found within an energy range of ã 15 kcal mol-1. The “lanternlike” C3v structure is the most favorable one for both complexes. Both molecules OM4H6 are stable to decomposition through all of the studied pathways. Chemical bonding in the OMk polyhedra containing two-, three-and four-coordinated oxygen atoms is discussed. © 1996 John Wiley & Sons, Inc.
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  • 159
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    International Journal of Quantum Chemistry 57 (1996), S. 721-728 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three stable tautomeric forms, dienol (DE), ketoenol (KE), and diketo (DK), of 2,2′-bipyridyl-3,3′-diol BP(OH)2 were found in this study, using the semiempirical AM1 and MNDO-PM3 and ab initio (4-31G basis set) methods. All calculations were carried out without any symmetry restrictions. There is a good agreement between the ab initio calculated and experimentally obtained structural parameters for the DE tautomer. Transition structures, corresponding to the DK → KE and KE → DE processes have also been found. On the basis of the results from the present work, an asynchronous (two-step) DK → KE → DE mechanism of the IPT reaction in BP(OH)2 is proposed. © 1996 John Wiley & Sons, Inc.
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  • 160
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    International Journal of Quantum Chemistry 57 (1996), S. 715-719 
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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 reaction mechanism of the α, α and α, β elimination of hydrogen fluorides from alkyl fluorides has been studied theoretically. For fluoroethane as a reactant, the transition state (TS) optimized at the level of the 6-31G** basis set shows that the α, β elimination proceeds via a four membered-ring TS with a barrier height 64.6 kcal/mol, while the α, α elimination, via a three-membered ring TS with a 83.7 kcal/mol barrier. Four substituents, CH3, CN, F, and NH2, were used to investigate the substituent effect of elimination by using the 3-21G basis set. The calculated barriers show that NH2-substituted alkyl fluorides favor both the α, α and α, β elimination and these two reactions would be expected to proceed simultaneously. © 1996 John Wiley & Sons, Inc.
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  • 161
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    International Journal of Quantum Chemistry 57 (1996), S. 729-733 
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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: Ab initio calculations were performed on the N-methylpyridones lithiated on the aromatic ring using a 6-31G* basis set. Whenever the lithium atom is on a carbon adjacent to the carbonylic group, a bridged structure is obtained where lithium is coordinated to both carbon and oxygen; these structures are the most stable isomers. © 1996 John Wiley & Sons, Inc.
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  • 162
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    International Journal of Quantum Chemistry 57 (1996), S. 735-740 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The system of [Pt(CO)(AuPPh3)8]2+ represents an example of the spheroidal 18-electron cluster which consists of 283 atoms, 817 real-valence orbitals, and, eventually, 1634 complex spin-orbitals. The above system was investigated by an MO-LCAO-SCF study at the all-valence (nonempirical) quasi-relativistic and relativistic CNDO/1 levels. Owing to the complexity of the system under study, instead of an orbital interaction diagram, the density-of-state functions were generated. Their projections, obtained through net orbital populations, bear information about the positioning of MOS and eventual interaction of the AOS composing individual MOS. The platinum 5d orbitals exhibit a mixing with those of gold and contribute to only a few HOMOS; the rest of gold 5d orbitals, however, are situated at lower energies. © 1996 John Wiley & Sons, Inc.
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  • 163
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    International Journal of Quantum Chemistry 57 (1996), S. 63-71 
    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 version of solutions in the form of an exponentially weighted power series is constructed for the two-dimensional circularly symmetric quartic oscillators, which reflects successfully the desired properties of the exact wave function. The regular series part is shown to be the solution of a transformed equation. The transformed equation is applicable to the one-dimensional problem as well. Moreover, the exact closed-form eigenfunctions of the harmonic oscillator can be reproduced as a special case of the present wave function. © 1996 John Wiley & Sons, Inc.
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  • 164
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    International Journal of Quantum Chemistry 57 (1996), S. 89-94 
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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: A kinetic energy analysis of total energy differences in 115 atomic multiplet states is presented. We show by numerical restricted Hartree - Fock calculations that there is a reasonably accurate linear relationship between the kinetic energy of the electrons in open subshells and the total energy within a manifold of states arising from the same spn or s2pn (n = 2,3,4) electron configuration in main-group atoms. © 1996 John Wiley & Sons, Inc.
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  • 165
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    International Journal of Quantum Chemistry 57 (1996), S. 775-780 
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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: It has already been shown that the use of the localized many-body perturbation theory (LMBPT) makes its possible to calculate the interaction energy at the correlated level in a straightfoward way. In this article, we show that the correlated part of the interaction energy, furthermore, can simply be decomposed into dispersion and charge-transfer contributions using the LMBPT scheme. The CH2O + NH3 model system was chosen for the above study. © 1996 John Wiley & Sons, Inc.
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  • 166
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    International Journal of Quantum Chemistry 59 (1996), S. 57-69 
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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: A comparison of sixth-order Møller-Plesset perturbation energies (MP6) with the corresponding full configuration interaction (FCI) energies shows that in the case of equilibrium geometries MP6 values differ by just 1.7 mhartree. MP6 correlation energies turn out to be important for systems with oscillatory convergence behavior as well as for systems with considerable multireference character. Contributions from pentuple (P) and hextuple (H) excitations are mostly positive and smaller than 1 mhartree in the cases investigated. Initial oscillations in the convergence behavior of a MPn series result from positive fifth-order T energy contributions to the correlation energy. In these cases, MP6 correlation energies are relatively large (T contributions dominate the total correlation energy) and absolutely necessary when estimating the convergence limit of the MPn series. MP6 is an O(M9) method and, therefore, can only be used for relatively small electron systems. More economic alternatives are given by the approximated MP6 methods MP6(M8) and MP6(M7), which involve O(M8) and O(M7) operations, respectively. According to calculated absolute and relative energies, MP6(M7) is an attractive alternative to full MP6 because it offers reasonable sixth-order results for cost comparable to those of MP4. © 1996 John Wiley & Sons, Inc.
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  • 167
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    International Journal of Quantum Chemistry 59 (1996) 
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  • 168
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    International Journal of Quantum Chemistry 59 (1996), S. 1-2 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 169
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    International Journal of Quantum Chemistry 57 (1996), S. 465-470 
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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 results of semiempirical calculations on the heterofullerence C24N4 and its boron-nitrogen analogs B6N10C12 (the smallest CBN ball) are presented in accordance with considerations of chemical bondings and geometries. The structures and relative properties of the 28-atom cages, C28, C24N4, and B6N10C12, were investigated using semiempirical methods contained in the MOPAC program. C24N4 and B6N10C12 have Td and C3 symmetry, respectively. B6N10C12 has four geometric isomers, and their optimized structures were determined. The ionization energies, vibrational frequencies, IR intensities, and various thermodynamic properties (including entropy and heat capacity) for C24N4 and B6N10C12 were calculated by the MNDO method. The evidence suggests that a B6N10C12 cage is more stable than that of C24N4. Possible experimental methods to prepare B6N10C12 are also proposed. © 1996 John Wiley & Sons, Inc.
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  • 170
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    International Journal of Quantum Chemistry 58 (1996), S. 399-406 
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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 coupling between the NH hydrogen inversion and the CNC bending modes of dimethyl-amine (DMA) is analyzed from ab initio calculations. For this purpose, the vibrational Hamiltonian is defined from the symmetry properties of the kinetic and potential operators. The two fundamental frequencies are determined with the Möller-Plesset perturbation theory up to four order (MP4). Calculated band positions are compared with data derived from other models in one dimension that reveal the strong effect of the interaction terms on the inversion bands. The frequencies of two components of the wagging fundamental are obtained to be 794.3 and 793.4 cm-1 with the two-dimensional model, in good agreement with experimental data. © 1996 John Wiley & Sons, Inc.
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  • 171
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    International Journal of Quantum Chemistry 58 (1996), S. 407-418 
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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: Second-order hyperpolarizabilities (β) and gas-phase transition energies (E) of the para-disubstituted benzene, stilbene and azobenzene derivatives are calculated by the semi-empirical self-consistent-field molecular orbit (SCF MO) method in the Pariser-Parr-Pople (PPP) approximation using a sum-over-state's procedure with singly excited states. The calculated zero-frequency β (β0VEC) values for all the derivatives are directly proportional to their 1/E2 values. The usefulness of mixing ethylenic and azo bridges in long π-conjugated molecules is proposed in order to obtain large β values, by taking the difference between the electronic natures of these bridges into account. © 1996 John Wiley & Sons, Inc.
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  • 172
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    International Journal of Quantum Chemistry 58 (1996), S. 419-429 
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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: Ab initio calculations of dynamic structure factors are performed with a methodology that requires the crystalline density matrix determined by the periodic Hartree-Fock CRYSTAL code. Amplitudes and orientations of the atomic vibrations are fixed by the mean-square displacements available in the literature. This method, which is here upgraded so as to satisfy normalization conditions, gives rise, for any temperature T, to a “dynamic” density matrix different from the static one. An application to the calculation of the dynamic structure factors of magnesium difluoride (MgF2), which has anisotropic mean-square displacement tensors, is proposed and compared with experiments at 298 K. © 1996 John Wiley & Sons, Inc.
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  • 173
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  • 174
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    International Journal of Quantum Chemistry 58 (1996), S. 431-439 
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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 ring-shaped Hartmann potential \documentclass{article}\pagestyle{empty}\begin{document}$$V = \eta \sigma ^2 \varepsilon _0 \left({\frac{{2a_0 }}{r} - \frac{{\eta a_0^2 }}{{r^2 \sin ^2 \theta }}} \right)$$\end{document} was introduced in quantum chemistry to describe ring-shaped molecules like benzene. In this article, fundamental concepts of supersymmetric quantum mechanics (SUSYQM) are discussed. The energy eigenvalues and (radial) eigenfunctions of the Hartmann potential are subsequently rederived using the techniques of SUSYQM. © 1996 John Wiley & Sons, Inc.
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  • 175
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    International Journal of Quantum Chemistry 58 (1996), S. 441-451 
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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: We study an example of a generalized Friedrichs model, in which a continuous-continuous coupling produces a pair of resonances as branch cuts of the analytic continuation of the reduced resolvent of the perturbed Hamiltonian to the second sheet of the Riemann surface associated to a transformation of the type w = z + α √z. To define the perturbation, we use the theory of self-adjoint extensions of symmetric operators proposed by the group of St. Petersburg. © 1996 John Wiley & Sons, Inc.
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  • 176
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    Topics: Chemistry and Pharmacology
    Notes: The dependence of the intramolecular charge transfer through the σ-electron systems of substituted hydrocarbons on the spatial arrangement of the X(SINGLE BOND)Cα and Cβ(SINGLE BOND)Cγ bonds has been studied using the perturbation theory for the one-electron density matrix (DM). Analytical expressions for the populations of the orbitals pertinent to the cis- and trans-bonds with respect to the X(SINGLE BOND)Cα bond have been obtained and analyzed. The Hamiltonian matrix elements determining the predominant direction of the above-defined charge transfer (cis or trans) have been revealed. The electron-accepting and electron-donating substituents (X) have been considered separately and the dependence of the resulting charge-transfer direction on the properties of substituent has been established. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 58 (1996), S. 461-469 
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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: A possibility of stabilization in an external homogeneous electric field on the HeH2(21 A′) metastable state relative to its nonadiabatic decay into the 1 1A′ state is investigated theoretically. If the lowest vibrational levels of the HeH2 (2 1A′) state might be stabilized, one would try to detect the radiative transition 1 1A′ ← 2 1A′ from these levels and thus to verify experimentally the existence of the HeH2 (2 1A′) state. It is shown that the separation between the potential energy surfaces (PESs) of the states in question and the energy of their avoided crossing point do increase in the field. Moreover, it is shown that in the fields with strength up to 107 V/cm the nonadiabatic decay of the 2 1A′ state along one of the vibrational modes may be closed. However, along two other modes such decay still may occur. It is noted nevertheless that the question of such stabilization possibility in inhomogeneous fields (i.e., in ionic crystal or on its surface) is still open. The effect of the field was taken into account within an analytic two-level model. The unperturbed PESs were calculated by the CASSCF method. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 58 (1996), S. 471-485 
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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: Using the ab initio coupled cluster method, correlation energies were calculated for a number of molecules composed of first-row atoms. The results of computations can be fitted rather well with simple analytic formulas. The main result of the present investigation is that intraatomic part of the correlation energy is proportional to sum of squares of valence electron charges on atoms composing the molecule. The proportionality coefficient depends on the basis set used. Independently, the approximations were introduced which allow for good estimation of intraatomic correlation energy by using M øller-Plesset second-order perturbation calculations only. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 58 (1996), S. 487-496 
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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: Temperature dependence of the experimental photoemission spectra of the high-Tc superconductors have been interpreted and simulated by the recently formulated molecular nonadiabatic theory of the electron-vibrational interaction. The change of the spectral feature below Tc, particularly formation of the narrow, near Fermi surface (FS) edge, peak separated by the dip from a broad spectral band at higher binding energies has been shown to be the consequence of the nonadiabatic electron-phonon coupling and corresponding properties of the nonadiabatic polarons. Simulation yields very good fit to the experimental temperature-dependent spectra, with the calculated value of the gap 30.5 meV. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 58 (1996), S. 497-515 
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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: Crystal-orbital calculations on the three-dimensional disodium deoxyguanosine-5′-monophosphate tetrahydrate, disodium uridine-3′-monophosphate tetrahydrate, monosodium monoprotonated deoxyadenosine-5′ monophosphate hexahydrate, and disodium deoxycytidine-5′ -monophosphate heptahydrate crystallohydrates were carried out in a Hartree-Fock all-valence-electron approximation using the CRYSTAL92 routine package. The first, the second, and the last compounds were found to be insulators, whereas the third one should be considered a two-dimensional semiconductor. The structural basis for these findings is discussed, with special reference to the semiconductivity and intermolecular interactions in solid-state samples of polymeric nucleic acids. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 57 (1996), S. 913-918 
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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: In the present article, polymethine and annulene electronic spectra are described in a unique way by means of the Pariser-Parr-Pople method. The analytical expressions were derived for the first π-electron transitions energies. The character of the annulene spectra, like the character of the open-chain spectra, was shown to be dependent on the ratio between electron and site numbers. The Dahne's triad theory and the existence of finite energy gap in polymethines are discussed. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 57 (1996), S. 929-941 
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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 potential energy surfaces of β-hydroxypropionic acid and 3-aminopropionamide have been investigated by means of RHF/4-31G calculations. Structures, reaction paths for internal rotations, and the respective energy barriers are reported. The influence of the various intramolecular interactions on structural and energetical properties is shown and compared to the results previously obtained for β-alanine. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 57 (1996), S. 17-33 
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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: An analysis of the structure of the optimized effective Kohn-Sham exchange potential vx and its gradient approximations is presented. The potential is decomposed into the Slater potential vs and the response of vs to density variations, vresp. The latter exhibits peaks that reflect the atomic shell structure. Kohn - Sham exchange potentials derived from current gradient approaches for the exchange energy are shown to be quite reasonable for the Slater potential, but they fail to approximate the response part, which leads to poor overall potentials. Improved potentials are constructed by a direct fit of vx with a gradient-dependent Padé approximant form. The potentials obtained possess proper asymptotic and scaling properties and reproduce the shell structure of the exact vx. © 1996 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 57 (1996), S. 53-61 
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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 D-dimensional Coulomb system serves as a starting point for generating generalized atomic shells. These shells are ordered according to a generalized Madelung rule in D dimensions. This rule together with an Aufbau Prinzip is applied to produce a D-dimensional periodic table. A model is developed to rationalize the ordering of the shells predicted by the generalized Madelung rule. This model is based on the introduction of a Hamiltonian, invariant under the q-deformed algebra Uq(so(D)), that breaks down the SO(D + 1) dynamical symmetry of the hydrogen atom in D dimensions. The D = 2 case (Flatland) is investigated in some detail. It is shown that the neutral atoms and the (moderately) positive ions correspond to the values q = 0.8 and q = 1, respectively, of the deformation parameter q. © John Wiley & Sons, Inc.
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  • 185
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    International Journal of Quantum Chemistry 57 (1996), S. 79-87 
    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 transition electric dipole moments between low-lying valence states of NH+ are calculated by an ab initio effective valence-shell Hamiltonian (Hv) method. The Hv calculated transition moments are found to be in good agreement with those by other accurate ab initio methods. The spontaneous emission probabilities for the A2- → X2Π, B2Δ → X2Π, and C2-+ → X2Π transitions of NH+ are computed. Also, radiative lifetimes for A2∑-, B2Δ, and C2∑+ states are all theoretically determined using the potential energy functions by Hv. Also, the Hv results are well compared with those computed using the Morse potentials and the rkr potentials which are obtained from experimental data. © 1996 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 186
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    International Journal of Quantum Chemistry 57 (1996), S. 157-172 
    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 size-extensive quadratic CI method with single(s), double(D), and triple (T) excitations, QCISDT, is compared with QCISD, QCISD(T), CCSDT-n, and CCSDT. It is shown that QCISDT results are more accurate than are either QCISD or QCISD(T) results. In particular, QCISDT turns out to be more stable than are QCISD and QCISD(T) in cases with considerable multireference character. QCISDT and CCSDT results are of similar accuracy with slight advantages for the former method. Since QCISDT is much easier to implement on a computer than is CCSDT, it is an attractive alternative to CCSDT. © 1996 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 187
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    International Journal of Quantum Chemistry 57 (1996), S. 173-182 
    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 remarks on the meaning of stopping power and of its magnitude. More precisely, the first set of remarks concerns the connection of stopping power with elements of particle-transport theory, which describes particle transport and its consequences in full detail, including its stochastic aspects. The second set of remarks concerns the magnitude of the stopping power of a material and its relation with the material's electronic structure and other properties. © 1996 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
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  • 188
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    International Journal of Quantum Chemistry 57 (1996), S. 327-341 
    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: The idea of dilation, or dilatation, analyticity with respect to complex scaling of the interparticle distances for nonrelativistic atomic or molecular electronic Hamiltonians is now over 20 years old and the first major reviews are just over 10. The method continues to be a fruitful source of new theoretical and computational results. Under the scale transformation r → reiθ, the usual “spectrum” of bound states is exactly preserved, and scattering continua are “rotated” off the real axis by an angle of -2Re(θ) about their respective thresholds. Useful features of this transformation are (1) that resonances are exposed, and, thus, (complex) resonance eigenvalues are easily calculated as the wave functions are L2, and standard results of Kato-type perturbation theory can thus be applied to them; (2) that utilization of this technique to study atoms in ac and dc fields was an early, and still evolving extension of the original theory; and (3) the fact that the “continua” are rotated off the real energy axis allows scattering information to be extracted from computations carried out entirely L2 in bases as the usual resolvents of scattering theory are no longer singular on the real axis. After a brief survey of the technique and its applications, these ideas are illustrated by discussion of positive energy-bound states and resonances, the extension of the theory to include the dc Stark effect, and a review of the resolution of the initially perplexing problem of atomic and molecular bound states in continua. These theoretical results are followed by a discussion of some very recent computational results, allowing computation of atomic partial photoionization cross sections with no specific coordinate space enforcement of boundary conditions, a highly advantageous situation for calculation of the partial cross section for three-body breakup, as in the process ℏ ω + He → He2+ + e- + e-. © 1996 John Wiley & Sons, Inc.
    Additional Material: 11 Ill.
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  • 189
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    International Journal of Quantum Chemistry 57 (1996), S. 361-367 
    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 orthogonal group function approach, as based on the Huzinaga equation, is extensively applied in reduced basis frozen-core calculations. Although the theory is developed for orthogonal electronic groups, the use of reduced basis sets prevents strict orthogonality and the formalism is complemented to take, partially, into account nonorthogonality (projection factors, projection energy). In the present article, an alternative to this approach, based on the nonorthogonal formalism, is proposed. An orbital equation is derived from the Adams-Gilbert equation and the energy is evaluated according to a recent proposal based on the power-series expansion of the overlap energy. A comparative overview of the orthogonal and nonorthogonal formalisms is presented and the results of reduced basis frozen-core calculations as obtained with the two methods are compared. It is found that the nonorthogonal formulation predicts equilibrium geometrical parameters in some cases similarly and, in other cases, slightly better than does the orthogonal one. Based on this observation and on the fact that the nonorthogonal formulation is exempt from empirical parameters (projection factors), it is concluded that the nonorthogonal formalism represents an appealing alternative in reduced basis frozen-core calculations. © 1996 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 190
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    International Journal of Quantum Chemistry 57 (1996), S. 377-389 
    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 recently introduced set of quadratic ionic and covalent valence indices from changes in the pair-diagonal part of the molecular (Hartree-Fock) two-electron density matrix, ΔΓ(2)(a, b) (orthogonal atomic orbital [OAO] basis set), relative to separated atoms limit (SAL), is extended to cubic valence numbers; they are calculated from the corresponding changes in the three-electron density matrix, ΔΓ(3)(a,b,c). The two- and three-electron indices are given by the corresponding contributions to ΔΓ(2), and ΔΓ(3), which are quadratic and cubic in terms of relevant changes in the one-electron density matrix. The new valence measures are partitioned into one-, two-, and three-center contributions comprising purely ionic (covalent) and mixed ionic-covalent terms. For integral OAO occupations in the SAL, the sum of all three-electron contributions vanishes exactly in the UHF approximation; for fractional occupations, they give rise to a rather small correction to the overall two-electron valence index. The properties of cubic valence numbers are tested on a model three-orbital description of a symmetric (ABA), collinear transition state and on the OQ2 (Q = 1,0, -1) and O2(bond)H+ systems. Preliminary results for B2H6 are also given. © 1996 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 191
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    International Journal of Quantum Chemistry 57 (1996), S. 449-452 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The purpose of the present work was twofold: First, a lower bound of Gálvez and Dehesa to the Thomas-Fermi kinetic energy was investigated for the two-electron systems H-, He, Li+, B3+, O6+, Ne8+, and Mg12+. Second, a conjecture of Lieb, relating the kinetic energy to the Thomas-Fermi kinetic energy, was examined. For both investigations, the analytical approximations of Benesch to the radial electron densities of the two-electron systems were used. These approximations are based on the explicitly correlated (Hylleraas-type) 20 variational parameter wave functions of Hart and Herzberg. © 1996 John Wiley & Sons, Inc.
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  • 192
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    International Journal of Quantum Chemistry 58 (1996), S. 109-119 
    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 ab initio CASSCF, CASPT2, and DFT computations have been performed on three different model systems which emulate the oxygenated active site of hemocyanin (a Cu+ - Cu+ dimer that binds oxygen as peroxide to form oxyhemocyanin). The three models differ in the number of the ammonia molecules (0, 4, and 6 molecules, respectively) which emulate the real histidine metal ligands of the protein matrix. While the CASSCF computations indicate that the ground state wave function of the oxyhemocianin active site is in all cases a singlet, the CASPT2 and the DFT approaches provide a significantly different description and suggest that the greater stability of the singlet versus the triplet state (experimentally observed) is not an intrinsic property of the oxygenated form of the hemocyanin active site but depends on the presence of ligands on copper atoms. These results indicate that the dynamic correlation contributions (included in the CASPT2 and DFT methods) are essential to obtain a proper description of these systems that cannot be correctly emutated using models where metal ligands are not included. © 1996 John Wiley & Sons, Inc.
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  • 193
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    International Journal of Quantum Chemistry 58 (1996) 
    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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  • 194
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    International Journal of Quantum Chemistry 58 (1996), S. 161-173 
    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: Magnetic and optical properties of transition-metal complexes are governed by the ground state and the low-energy excitation spectrum of the d-shell of the central transition metal ion. These spectra are successfully fit to the crystal field theory. We present here an account of the effective Hamiltonian method recently developed to calculate the ground state and the excitations of the d-shells of transition-metal complexes and report the results of its application to some complexes of particular interest. © 1996 John Wiley & Sons, Inc.
    Additional Material: 8 Tab.
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  • 195
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    International Journal of Quantum Chemistry 58 (1996), S. 175-184 
    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 problems of semiempirical quantum chemical calculations of (a) spin densities in paramagnetic organometallics, (b) hydrogen bonds, and (c) bond energies and the structure of transition-metal compounds are discussed. Some modifications of the existing semiempirical quantum chemical method are presented. An extended NDDO approximation has been developed. This scheme includes explicit symmetric orthogonalization of the core Hamiltonian and the use of Hellmann's effective core potential for core-electron interaction. © 1996 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
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  • 196
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    International Journal of Quantum Chemistry 58 (1996), S. 251-265 
    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: Electronic properties of a water molecule embedded in a water droplet are studied in the framework of the generalized self-consistent reaction field approach, using ab initio Hartree-Fock and configuration interaction wave functions. Electrostatic and inductive effects of the surrounding water molecules were calculated with the help of configurations drawn from a classical molecular dynamics simulation. Basis-set effects and solute-solvent interaction operator representation are examined. Embedding energies and liquid-phase multipole moments obtained from the present mixed quantum-classical model are compared with corresponding quantities for purely classical water models. © 1996 John Wiley & Sons, Inc.
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  • 197
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    International Journal of Quantum Chemistry 58 (1996), S. 283-295 
    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: Various approaches for surface simulation are described. They are based on free, saturated, and embedded clusters, as well as periodic models. The features of semiempirical methods are reviewed and ideas for their improvement are discussed. Special features of the structure and stability of clusters are presented which are suitable for the transition to the solid state. Linear and nonlinear relationships for binding energies and bond lengths in dependence of the average coordination number are discussed. Finally, a systematic way for the simulation of adsorption at ion crystal surfaces by model clusters is suggested. © 1996 John Wiley & Sons, Inc.
    Additional Material: 11 Ill.
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  • 198
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    International Journal of Quantum Chemistry 57 (1996), S. 3-6 
    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 kinetic and the exchange energy functionals are expressed in the form T[ρ] = CTF∫ drρ5/3(r)ft(s) and K[ρ] = CD∫ drρ4/3(r)fK(s), where CTF = (3/10)(3π2)2/3 and CD = -(3/4)(3/π)4/3 are the Thomas-Fermi and the Dirac coefficients, respectively, and s = |∇ρ(r)|/Csρ4/3(r), with Cs = 2(3π2)1/3. These expressions are used to perform a comparison of fT(s) and fK(s) in terms of their generalized gradient expansion approximations. It is shown that fκ(s) and is congruent to fT(s) in the range characteristic of the interior regions of atoms and many solids and that the second-order gradient expansion of the kinetic energy provides a rather reasonable approximation to the generalized gradient expansion approximation of both the kinetic and the exchange energy functionals. © 1996 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 199
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    International Journal of Quantum Chemistry 57 (1996), S. 229-233 
    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: Restricted open-shell Hartree-Fock and unrestricted Hartree-Fock calculations of the electric-field gradient in atoms B, N, O, Al, S, and Cl were performed by relieving the spherical symmetric constraint. The Sternheimer's core polarization effect is then automatically taken into account. The orbitals produced by the axial symmetric self-consistent field are found to have axial symmetry of s-d and p-f mixing types. However, the nonequivalence of the three p orbitals also gives rise to ambiguity. © 1996 John Wiley & Sons, Inc.
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  • 200
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    International Journal of Quantum Chemistry 58 (1996), 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
    Notes: No Abstracts.
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