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  • Atomic, Molecular and Optical Physics  (441)
  • 2010-2014
  • 2005-2009
  • 1990-1994  (441)
  • 1985-1989
  • 1970-1974
  • 1992  (441)
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  • 2010-2014
  • 2005-2009
  • 1990-1994  (441)
  • 1985-1989
  • 1970-1974
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  • 1
    ISSN: 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 second-order Rayleigh-Schrödinger energy correction to the Born-Oppenheimer potential energy due to the spin-orbit interaction can be expressed as a linear response function evaluated at zero frequency. We have calculated this energy contribution to the Cr2 singlet ground state X1 Σ+g potential energy function for a multiconfiguration self-consistent field (MCSCF) wave function. The calculations show that the effect of spin-orbit interaction is small and of the same magnitude for the whole potential energy curve.
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  • 2
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hartree-Fock-Roothaan studies are reported for low-lying electronic states of metallic beryllium as modeled by a moiety of 135 beryllium atoms. The system corresponds to 16 coordination shells of a central Be with internuclear separations derived from the lattice constants of the bulk metal. The calculations become tractable by use of the full D3h symmetry of the system at both the integrals and self-consistent-field stages and by employing ab initio effective potentials for the 1s electrons of each beryllium atom. Ionization potentials, binding energies, orbital energies, electric field gradients, nuclear-electrostatic potentials, diamagnetic shielding constants, second moments, and Mulliken populations are calculated for selected electronic states. The calculated ionization potential for the lowest state agrees to within 10% of the experimental bulk work function. A density-of-states analysis for that state is reported and compared with band structure calculations.
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  • 3
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 4
    ISSN: 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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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 commercial computer algebra program, Mathematica, is used to generate the C matrix that characterizes our implementation of the Löwdin α-function method as applied to Slater-type orbitals. An example of a two-center overlap integral is done to show how the arbitrary precision capability of Mathematica can overcome severe cancellation errors encountered with programming in FORTRAN. This strategy is capable of being generalized to other multicenter molecular integrals. Mathematica programs are included.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 7
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 application of the Prüfer transformation to the Schrödinger equation leads to a nonlinear first-order differential equation. The advantage of this transformation is demonstrated in the calculation of the energy spectrum of the H2+ ion. In the second section, the classical problem is recalled and the transition to Prüfer transformation prepared. The definition of this transformations is contained in the third section. In the final paragraphs, the numerical results concerning the eigenvalues of H2+ energy are presented.
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  • 8
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is demonstrated that the maximum overlap symmetry molecular orbital method can be used for optimization of molecular geometries and calculation of vibrational frequencies by adding a two-body repulsive energy term and a modification of the Wolfsberg-Helmholz formula. The obtained equilibrium geometries and vibrational frequencies are on the whole in good accordance with experimental data, which shows that the basic idea using the method to optimize molecular geometries and to calculate vibrational frequencies is reasonable.
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  • 9
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Second-order multiconfigurational self-consistent field (MCSCF) calculation has been programmed on the basis of CNDO/INDO molecular orbital bases, in which the configuration space employed is restricted within pair-excitations. Test calculations have been carried out for 17 small molecules. All the MCSCF ground states of these molecules have been successfully converged to their respective optimal states by employing a simple weighting scheme. This procedure provides a great savings in computer time. The MCSCF calculation on azetidine required only 27 min on a HITAC M-680H. The MCSCF energies of HF, F2, and BH show improved behaviors up to large atomic distances (∼7au).
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  • 10
    ISSN: 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 ab initio electronic structure study is presented of hydrogen-hydrogen interactions in an electronic environment perturbed by the presence of palladium atom clusters. In particular, we investigated changes in the interatomic potential when the atomic centers are trapped inside an fcc palladium octahedral hole and when they are separated from each other by a (111) plane of palladium atoms. The (111) plane was modeled with a cluster of three palladium atoms. Self-consistent field (SCF) level calculations were performed, and palladium atom pseudopotentials were employed to make the systems studied computationally tractable. For pairs of atoms placed within the octahedral hole, various lines of approach were considered [along the (100), (110), and (111) directions]. Lattice deformations and electronic excitations were examined for their effect on the interatomic potential.
    Additional Material: 13 Ill.
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  • 11
    ISSN: 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 algebraic approach is proposed to calculate the Franck-Condon factors for the Morse potential of diatomic molecules. The Morse oscillator is approximated by means of a fourth-order anharmonic oscillator. In the second-quantized formalism, this anharmonic Hamiltonian is diagonalized by way of the Bogoliubov-Tyablikov transformation. The Franck-Condon factors are estimated using the harmonic frequency equivalent and the recurrence relations for the Franck-Condon factors of the harmonic oscillator. Overlap integrals are shown for three band systems and compared with values calculated with an RKR potential. Excellent agreement is achieved.
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  • 12
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic structure of 5α-androstane, the parent hydrocarbon of the hormonal steroids, has been computed by ab initio SCF methods in an STO-3G basis. The results are compared with existing MNDO computations and are used to discuss long-range electronic interactions between distant substituents that might be appended to rings A and D of 5α-androstane. It is thought that these interactions are mediated by the ribbonlike MO'S of the parent molecule.
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  • 13
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general solution to the one-dimensional time-independent Schrödinger equation is derived using the properties of the Laplace transform. The derivation assumes that the potential function is real and that it can be expressed as a Fourier series with a finite number of terms, which includes, but is not limited to, equations of the type of Hill's or Mathieu's equations. In the case where the Laplace transform of the potential function contains only simple first-order poles, then a complete closed-form solution is derived. In other cases, an approximate solution is derived and the symbolic algebra program MACSYMA is used to carry out the inverse Laplace transform. The MACSYMA implementation of this general solution is discussed and is applied to a sample problem, namely, the solution of eigenvalues of Mathieu's equation. Comparison is made between the eigenvalues obtained by this method and those obtained by another analytical solution of this equation, by Hill's method.
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  • 14
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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: Dynamic response of a helium atom in an intense laser field has been studied within a quantum fluid density functional framework. Several time-dependent quantities have been calculated by solving a recently derived generalized nonlinear Schrödinger equation.
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 16
    ISSN: 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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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 18
    ISSN: 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: Starting out with the electron gas, we make a survey of the reasons for the singularity in the derivative of the orbital energy with respect to the wave number at the Fermi level for a realistic extended metallic system. Some properties of the occupation function are reviewed and it is pointed out that the direct reason for the singularity resides in a divergent lattice sum originating in the exchange part of the orbital energy. Numerical aspects are discussed, in particular with reference to the difficulty in detecting this singularity in actual computations.
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  • 19
    ISSN: 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-phase transition associated with the singlet instability problem of the Hartree-Fock solution of the extended system is studied employing the metallic trans-polyacetylene. Concerning the eigenstates of the stability matrix set up for the crystal orbitals at the Fermi level, classification of the electronic phases is attempted and the origin of their emergence in relation to the interelectronic interaction is discussed.
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  • 20
    ISSN: 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: Silicon is taken as a test system for assessing present-day feasibility of calculations for crystalline solids of near-Hartree-Fock quality. The calculations have been performed using CRYSTAL, an ab initio Hartree-Fock crystalline-orbital LCAO program for periodic systems. The influence of the computational parameters that control the truncation of infinite sums on the results has been investigated; it is shown that a reasonable accuracy (numerical errors on total energy per atom below 10-3 a.u.) can be obtained while keeping the computational burden within manageable limits. The effect on the results of basis-set size and quality is discussed. A number of basis sets have been tested, from minimal to relatively extended sets (28 atomic orbitals per atom). The quality of the wave function has been checked using variational criteria and also through a comparison with experimental data, such as equilibrium geometry, bulk modulus, electron charge density, and electron momentum distribution. For the latter quantities, which are a measure of the accuracy of the one-electron density matrix, the best basis sets provide agreement with experiment that is almost within the experimental error. The correlation energy has been estimated using nonlocal density functionals, based on the one-electron density matrix: After this correction, the atomization energy agrees with experiment to within 2%. The generalization of the above analysis to other crystals is briefly discussed.
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  • 21
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    New York, NY : Wiley-Blackwell
    ISSN: 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 paper describes a group theoretical classification of the unrestricted Hartree-Fock (UHF) solution of the extended Hubbard model including nearest-neighbor repulsion and exchange interactions. We derived systematically all types of the broken symmetry solution with a single-order parameter for the ordering vector Q = (π,π). It is shown that there are four nonmagnetic states: a charge density wave (CDW), a charge current wave (CCW), and two bond-order waves (BOW), and five magnetic states: an ordinary antiferromagnetic state (AF), a spin current wave (SCW), two axial spin bond-order waves (ASBOW), and an helical spin bond-order wave (HSBOW). We showed that these phases can be derived naturally from the irreducible decomposition of the Fourier transformed interaction and the self-consistent field (SCF) condition. Some numerical results are given for the half-filled case.
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  • 22
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Methods are discussed for iterating the many-particle Schrödinger equation starting from Hartree-Fock wave functions. It is shown that when the Hartree-Fock wave function is expressed as a linear combination of the appropriate basis functions in hyperspherical coordinates the hyperangular integrals needed for iteration can be evaluated analytically. It is also shown that the first-iterated solutions contain terms corresponding to angular correlation.
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  • 23
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 survey is given of ab initio calculations for solids and polymers, which start from an SCF calculation and treat correlations by using local operators. The theory is formulated by employing a projection method. As regards applications, special attention is paid to elemental semiconductors and to polymers like polyethylene and polyacetylene. With the help of reduced Hamiltonians, insight can be gained into various correlation effects. This is demonstrated by a discussion of the problem of dimerization in polyacetylene. Finally, a formalism is described that allows for treating the effects of correlations on energy bands of solids. It is shown that qualitative new aspects of correlations enter when a quasiparticle in, e.g., a semiconductor is considered. The theory is applied to a calculation of the energy bands of silicon.
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  • 24
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    New York, NY : Wiley-Blackwell
    ISSN: 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 two main problems in applying common methods for electron correlation to large systems are (1) the steep dependency of the computational task with the size of the system and (2) the large and often prohibitive number of two-electron integrals that must be stored and retrieved during the calculation. The direct local correlation method eliminates both these problems. This method is a combination of the local correlation method that eliminates the steep computational dependency with the size of the system and extension of the direct SCF approach to electron correlation methods. The latter method eliminates external storage of the electron repulsion integrals. In this review, the direct local correlation approach and its computer implementation will be described in detail, including computational examples and comparisons to similar methods developed in other groups. The feasibility of future applications to extended systems is discussed.
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  • 25
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 26
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 27
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A formalism that describes the variation of the spectroscopic properties, De, Re, and ke, of homonuclear, diatomic molecules, with the number of molecular electrons has been developed. The theory describes the interrelation of these properties and predicts “critical” behavior in sequences of “isonuclear” and neutral molecules. Detailed calculations are possible with the help of experimental data in lieu of a deeper, dynamical theory of molecular behavior with respect to electron number. The present work points the way toward a first-principle's theory. © 1992 John Wiley & Sons, Inc.
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  • 28
    ISSN: 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 lower bounds for the nonrelativistic lowest triplet states of 4He and ∞He and the He isoelectronic series up to Z = 10 were calculated by the method of variance minimization using wave functions with logarithmic terms. Furthermore, the method of Lehmann/Goerisch was extended to self-adjoint operators with an essential spectrum. Both methods were compared for 4He. © 1992 John Wiley & Sons, Inc.
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  • 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 possible combination of two methods for the calculation of multicenter two-electron integrals using STO and B function basis sets is discussed. The first method (Method I), which is of approximate nature, is based on a simplified version of the so-called Σ-factorization method [A. W. Niukkanen and L. A. Gribov, Theor. Chim. Acta 62, 443 (1983)], where the radial part of the two-center one-electron density ρab(r) is represented as a sum of two radial functions fa(r) and fb(r), placed on two different centers a and b. After such a transformation, the calculation of the two-electron integrals boils down to the calculation of some type of Coulomb integrals. The second method (Method II), which calculates each integral separately to a given accuracy, is based on Mobius-type quadrature used for a three-dimensional integral representation for the two-electron integral of B functions [E. O. Steinborn and H. H. H. Homeier, Int. J. Quantum Chem. Symp. 24, 349 (1990)]. In Method I, the choice of the radial functions placed on the different centers has an essential influence on the final value of the multicenter integrals. In the present study of Method I, a rather simple approximation of the radial part was made that reproduced the qualitative behavior of the molecular integral curves as function of the geometry rather well. At the present state of development, Method I produces fast order-of-magnitude estimates that are useful for screening purposes, i.e., to decide which integrals have to be evaluated more accurately by other methods like Method II. Method II reproduces data given in the literature [R. M. Pitzer and D. P. Merrifield, J. Chem. Phys. 52, 4782 (1970)] correctly. © 1992 John Wiley & Sons, Inc.
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  • 30
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    New York, NY : Wiley-Blackwell
    ISSN: 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 intramolecular (internal) charge sensitivity analysis (ICSA) is presented in the molecular orbital (MO) resolution, with diagonal idempotency constraints being imposed on the MO occupations, n, which can then be considered as independent electron population variables of the system for the assumed fixed shapes of MOS. The standard closed-shell (C.S.) SCF MO framework is adopted with the energy function E(n) approximated by the average energy of a configuration expression to cover fractional MO occupations. The components of the gradient of the auxiliary energy function, including the constraint terms, with respect to n, identified as negative MO internal electron potentials, vanish identically for the ground-state c.s. configuration, whereas their responses to hypothetical intramolecular charge displacements assure the system stability. An application of the ICSA to the iterative SCF procedure is suggested. Illustrative results for the water molecule are reported and general properties of molecular normal orbitals (eigenvectors of the hardness matrix) are discussed. © 1992 John Wiley & Sons, Inc.
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  • 31
    ISSN: 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: Closely related structures, like esters and lactones, have vastly different physical properties. This is apparently due to differences in the intermolecular interactions. The intermolecular interactions of methyl acetate, β-propiolactone, ethyl acetate, and γ-butyrolactone have been studied using the AM1 semiempirical method. Some of the “arranged clusters” were also compared to possible covalently bound trimers and tetramers of β-propiolactone and γ-butyrolactone. © 1992 John Wiley & Sons, Inc.
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  • 32
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 33
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 34
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 35
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Local density functional theory (DFT-LDA) has been explored as a tool for obtaining the molecular electrostatic potential V(r), using the code DMol. We have presented and discussed DFT-LDA electrostatic potentials for a representative series of molecules: ethylene, benzene, formamide, cytosine, and 2,3,7,8-tetrachlorodibenzo-p-dioxin. V(r) results obtained with a double numerical plus polarization (DNP) basis set show the key features that are characteristic of the ab initio potentials of these compounds and suggest that this is a useful approach, especially for large molecules that are difficult to study by ab initio methods.
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  • 36
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Semiempirical methods were utilized in the computation of a fully optimized structure of bilirubin. Bond lengths and bond angles obtained using either AM1 or PM3 calculations showed excellent agreement with those obtained by X-ray diffraction. This indicated that molecular orbital methods satisfactory reproduced the complex conjugation found in bilirubin. Dihedral angles of the crucial “hinge” and the dihedral angles of the propionic acid side chains agreed well with those found by X-ray diffraction. Calculated hydrogen- bond parameters (distance and angles) showed substantial differences from experimental values, probably due to inherent weakness in the parameterization of the molecular orbital techniques. Conformational studies were carried out using AM1 by rotating the C9—C10 bond in 5° increments showed that the most stable structure exhibited a minimum at about 125° and exhibited a structure similar to those postulated from X-ray and NMR experiments. The hydrogen bonds showed remarkable tenacity during rotation of the C9—C10 bond and resisted breaking until the molecule was under extreme strain. © 1992 John Wiley & Sons, Inc.
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  • 37
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This paper provides a brief and systematic presentation of the basic principle and method of the maximum overlap symmetry molecular orbital (MOSMO) model and its application to simplification of molecular orbital calculation and to calculation of molecular structures and properties, together with some new results about the MOSMO calculation and new insights concerning the further extension of the principle and method. It has been shown that the theoretical method of the MOSMO model is very simple, reliable, and useful and can be employed to study the structure-property relation in even very large molecular systems. The numerical results obtained from the MOSMO calculation on various semiempirical molecular orbital approximation levels show that when the same parametrization, such as one of those employed in EHMO, CNDO/2, and HMO methods, is adopted, the MOSMOS are very close to the canonical molecular orbitals obtained from the customary LCAO method and the MOSMO calculation requires less computing time than does the LCAO method. The MOSMO calculation can be used for rapidly obtaining reasonably good molecular geometries, vibrational frequencies, and other properties of molecules by employing a simple improved semiempirical parametrization. Equilibrium geometries, vibrational frequencies, and other results are in good agreement with the experimental data and the results obtained from ab initio molecular orbital calculation. The basic calculational procedure of the MOSMO model can be extended further and has been employed to give some new results, to propose some new theoretical schemes and principles, and to introduce some new interesting theoretical problems that deserve to be studied further.
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  • 38
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: One long-range goal of our work has been the design of cannabinoid analgesics with reduced psychopharmacological liability. We review here the progress we have made toward this goal. When we began our work on the cannabinoids, no cannabinoid analgesic had been reported that did not also possess significant psychoactivity. We approached the cannabinoid problem by calculating molecular reactivity characteristics (MRCS) of template molecules for each activity: (-)-trans-delta-9-tetrahydrocannabinol (Δ9-THC) for psychoactivity, and (-)-9-nor-9-betahydroxyhexahydrocannabinol (9-nor-9β-OH—HHC) for analgesic activity. We then compared the MRCs of each template with the MRCS of a carefully chosen set of other cannabinoids (both active and inactive). Our focus in the calculation of MRCS was on accessible conformations of each subject molecule and on the molecular electrostatic potential generated in key planes by each accessible conformer. Our working hypothesis concerning the molecular basis of psychopharmacological activity in the cannabinoids was that activity is conferred by the set of molecular properties generated (1) by the orientation of the lone pairs of electrons of the phenyl group hydroxyl oxygen and (2) by the orientation of the carbocyclic ring and its C-9 substituent relative to this oxygen. Our working hypothesis concerning the molecular basis of cannabinoid analgesic activity was that activity is conferred by the set of molecular properties generated by the presence and relative location of two negative potential regions in the top half of the cannabinoid analgesic molecule. Our work thus far has revealed important steric and electronic features of the cannabinoids that may be used to enhance discrimination. We have shown that a simple structural feature, a C-9 substituent protruding into the α face of the molecule, can abolish or significantly diminish psychoactivity. We have also demonstrated that a free phenol group is not an absolute requirement for activity. Instead, we have found that compounds that do not possess free phenolic hydrogens, but that do maintain the orientation of the lone pairs of electrons of the “phenolic” oxygen toward the carbocyclic ring, exhibit selectivity toward analgesia.
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  • 39
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Semiempirical calculations of the α-2 receptor antagonist, Idazoxan, its methoxy derivative, and their respective cations and anions revealed the most stable conformers were significantly affected by electrostatic interactions. These studies showed the importance of the intramolecular electrostatic interactions between the oxygen of the benzodioxane ring and the nitrogens of the imidazoline ring in determining the global minima of these molecules. The NH bond of the imidazoline ring was attracted to the dioxane oxygen and caused the imidazoline ring to rotate to a position allowing close proximity of these groups. Favorable interactions between the oxygens and the imidazoline stabilized the axial conformer, whereas unfavorable interactions increased the stability of the equatorial isomer. AM1 and PM3 calculations sometimes predicted different global minimum conformers, indicating the need for caution in reliance on either method, particularly when hydrogen bonds play an important role in determining the structure. © 1992 John Wiley & Sons, Inc.
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  • 40
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The vibrational self-consistent-field approximation is used to calculate excited vibrational energy levels of the water molecule in hyperspherical coordinates. The calculations are made for a global realistic Sorbie-Murrell-type potential surface for which exactum quantum variational results are known for comparison. The coupled SCF equations are solved using the discrete variable representation (DVR) method, which allows computation of the coupled multidimensional integrals in a very simple and efficient way. The results are in good agreement with exactum quantum calculations and are more accurate than SCF energy eigenvalues obtained using normal mode coordinates.
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  • 41
    ISSN: 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 novel algorithm for computing the water/1-octanol partition coefficient, log P, of conformationally flexible molecules, has been investigated using calculations upon a number of uncharged, linear dipeptides. In this method (which appears to be the first to consider explicitly the effects of the population of accessible conformational minima in both phases), the partition coefficient for each dipeptide was calculated from the overall energy change associated with moving the relevant gas-phase conformational distribution into water and into 1-octanol. These energies were computed using solvation contributions based upon the solvent accessible molecular surface area and two sets of empirical parameters. In these initial studies, gas-phase conformational minima were generated using systematic search methods. While the standard error in the computed logP values was disappointing, reasonable agreement was observed between calculated and experimental logP values for the set of model dipeptides, especially when specific hydration interactions involving polar fragments were correctly included in the empirical solvation term. These results indicate that the physical basis of many correction factors employed in the ClogP algorithm for computing logP probably arise from neglect of the redistribution of conformer populations as flexible molecules partition between water and 1-octanol.
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  • 42
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The recently proposed topological approach to chemical reactivity in terms of the secondorder similarity index was applied to the detailed analysis of correlation effects in pericyclic reactivity. This analysis implies that (i) the electron correlation is generally more important in forbidden reactions than in the allowed ones; (ii) in contributing to the discrimination between the allowed and forbidden reactions, the Fermi correlation acts in parallel with the Coulomb one; and (iii) the so-called multibond reactions are more sensitive to electron correlation than are the electrocyclic ones.
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  • 43
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 44
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 45
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 4 Ill.
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  • 46
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present a method for including the Breit interaction in relativistic self-consistent field calculations for closed-shell molecular systems using atomic basis spinors of kinetically balanced Gaussian-type functions. The method extends the formalism described in a previous paper [A. Mohanty and E. Clementi, Int. J. Quantum Chem. 39, 487-517 (1991)] that dealt with the two-electron effect due to Coulomb interaction only. It is shown that both frequency-dependent and frequency-independent Breit interactions can be treated on equal footing, and the corresponding matrix elements are evaluated following the well-known Fourier transform technique applied to electron repulsion integral evaluation in nonrelativistic molecular calculations.
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  • 47
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We review theory and computational aspects for analysis of the core electron shake phenomenon.
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  • 48
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Equations of motion on the levels of classical mechanics, hydrodynamics, and wave mechanics are analyzed and existing connections are shown. The manifestation of particle and wave aspects on the different levels are investigated. Dissipative frictional forces are included and a nonlinear wave mechanical equation for these systems can be found. It follows the discussion of an alternative to the time-dependent Schrödinger equation (SE) in order to obtain the dynamical information on particle and wave aspects in a different form. This can be achieved by means of a Newtonian equation for a complex variable in connection with a conservation law for a nonclassical angular momentum-type quantity. With the help of this complex variable it is also possible to develop a Hamiltonian formalism for the wave aspect contained in the SE, the fluctuation, or uncertainty of position and momentum. In this case the Hamiltonian function is equivalent to the difference between the mean value of the Hamiltonian operator and the classical Hamiltonian function. Possible extensions and applications are mentioned.
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  • 49
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A brief survey is given of the abstract contentless Boolean algebra and its realizations to logic, class theory, and electric circuits as well as its applications to the software and hardware of the modern electronic computers. It is shown that three relations, 1 + 1 = 1, 1 + 1 = 0, and 1 + 1 = 10, correspond to three different realizations of the algebra, and that they, hence, represent an elementary example of a Gödelian branching point in the abstract Boolean algebra.
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  • 50
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio self-consistent field (SCF) molecular orbital calculations were carried out at the MP2/431G* level to determine the effect of substituent groups, —F, —NH2, —OH, —CH3, CH2=CH—, and H—C≡C - on the C≡P triple bond. Although there are considerable differences between the electronegativities of these substituent groups, a characteristic feature found in phospha alkyne derivatives is the weak sensitivity of the C≡P bond length to the substituent at the carbon atom. The molecular electrostatic potential analysis clearly reflects the strength of the electronegativity of these substituent groups and their ability to attract the polarizable charge from the C≡P triple bond. MEP analyzes further indicates that these derivatives of phospha alkynes are most likely to attract incoming electrophiles to the π-electron region of the C≡triple bond. Transition metals are coordinated “sideways” with phospha alkynes, and in this configuration, the lone-pair electrons of the phosphorus atom are unlikely to interact with the metal. © 1992 John Wiley & Sons, Inc.
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  • 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: This work examines the possibility of finding an electron-hole and an electron-pair simultaneously in a π-system substituted by an electron-donating (NH2) and/or electron-withdrawing (NO2) group. The contributions of various ionic [(+), ( ↑ ↓ )] structures are calculated from ab initio SCF-CI wave functions, using a recently developed general multielectron population analysis. The molecules studied are two monosubstituted ethylenes, aminoethylene and nitroethylene, and a disubstituted ethylene, the 2-nitroethenamine (push-pull ethylene) in its two configurational forms. The influence of the NH2 and/or NO2 group in delocalization and ionic (vs. covalent) character of the C=C double bond are investigated, along with examining the experimental chemist formalism of electron-pair “displacements” in several resonance structures. Analysis of mutual dependence of an electron-hole and an electron-pair, at short and long distances, leads to the conclusion that a push-pull π-system can stimulate the simultaneous existence of an electronhole and an electron-pair even for nonvicinal positions. The relationship between the electronpair distributions and contributions of the corresponding structures are also examined; the effects of electronic correlation are analyzed as well. © 1992 John Wiley & Sons, Inc.
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  • 52
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A systematic procedure has been developed to construct an electronic energy matrix for diatomics in the basis of antisymmetrized products of atomic wave functions represented as linear combinations of coupled momenta eigenfunctions. The exchange matrix element is expanded in powers of electronic interchange between atoms. General expressions of many-electron angular coefficients have been obtained for all types of products of one- and two-electron and overlap integrals in energy matrix elements. © 1992 John Wiley & Sons, Inc.
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  • 53
    ISSN: 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 developed Monte Carlo method is used to compute the 22S → 22P transition dipole moment of Li. This approach employs a guided Metropolis random walk with quantum Monte Carlo “side” walks to sample the required probability distributions. The transition dipole moment is employed to obtain the oscillator strength and excited-state lifetime. Our most accurately converged calculations yield an oscillator strength of 0.742(7) and excited-state lifetime of 27.41(35) ns. These results are in excellent agreement with precise experimental measurements of 0.742(1) and 27.29(4) ns, respectively. In addition, single-state expectation values are computed for both states. Monte Carlo parameters, such as the time step size and the convergence time, are varied in order to study their effect on computed results.
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  • 54
    ISSN: 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: Structural properties of cyclopropabenzene and its heteroanalogs are studied at the SCF 6-31G level of theory. It is found that the Mills-Nixon (MN) type of localization within the benzene ring is present in these systems. The origin of the bond fixation lies in rehybridization at the carbon junction atoms. π-electron bond orders usually follow changes within the sigma framework but can be misleading sometimes. It is shown that a judicious choice of heteroatoms can considerably enhance the MN effect. A refined and better definition of the MN effect is offered. Finally, present calculations and analysis of the results in terms of hybridization model and π-bond orders strongly indicate that experimental 4J(H—C—C—Me) proton-proton spin-spin couplings over four bonds can be used in identification of the MN effect only with extreme caution.
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  • 55
    ISSN: 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 recent theory of nonadiabatic effects in triatomic molecules is specialized to the four-state Renner-Teller and Jahn-Teller 1Πu/1Φg interactions and is then generalized by including the electronic spin and by considering the 2Λ Renner-Teller effect.
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  • 56
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure of silacarbonyl ylide in the singlet gound state is obtained by ab initio quantum chemical calculations. This proves the recent experimental observation of silacarbonyl ylide by the photolysis of oxasilitane [W. Ando, K. Hagiwara, and A. Sekiguchi, Organometallics 6, 2270 (1987)]. The dynamic stability of the silacarbonyl ylide is studied by the TCSCF/3-21G(*) method. The second-order Jahn-Teller effect is examined in connection with the active role of frontier orbitals. Configuration analysis is used to clarify characteristic electronic structure of the silylene-carbonyl interaction.
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  • 57
    ISSN: 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 heuristic fitting procedure to obtain an analytical potential function for describing a reactive potential energy surface in the neighborhood of the intrinsic reaction coordinate (IRC) has been developed. For discussion, the pairwise potential function form, ∑anr-n, is assumed in order to fit ab initio quantum mechanical calculations of intramolecular (or intrasupermolecular) interaction energies and its use is found advantageous because all the calculation can be carried out by the linear least squares method. Normal modes perpendicular to IRC are utilized to prepare an initial data base for the potential fitting in the neighborhood of IRC. Some trial molecular dynamics (MD) simulations are performed in order to check the fitted potential function and, unless they lead to reasonable energies within the tolerance assumed, their results are utilized to construct an improved data base (the dynamic sampling). The present systematic optimization procedure has been applied to the proton transfer reaction of the formamidine-water (FW) system. The normal mode analysis in both the transition state (TS) and the stable state (SS) regions suggests that the present fitted potential function can reproduce satisfactorily the Born-Oppenheimer (BO) adiabatic surface obtained by ab initio molecular orbital (MO) calculations. We conclude that our procedure works well for the chemical reaction molecular dynamics (CRMD) simulation.
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  • 58
    ISSN: 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 proposed ab initio method for calculation on many-electron molecular systems with the approximation of the inactive part of a molecule by a frozen molecular fragment was tested further in a case of the dissociation reaction of the C—F bond in n-fluoropropane. Results from the Hartree-Fock, multiple reference double-excitation configuration-interaction and second-order Møller-Plesset methods are presented. The reproduction of potential energy surfaces as well as the reproduction of electron density distribution are in excellent agreement with extended basis-set calculations. Different choices of fragments to be frozen have been examined.
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  • 59
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A previously proposed [Sutcliffe and Tennyson, Int. J. Quantum Chem. 29, 183 (1991)] body-fixed Hamiltonian is applied to AB2 systems in Radau coordinates with the x-axis embedded along the bisector of the angle. It is shown that by using a discrete variable representation for the angular coordinate it is possible to avoid singular regions of the Hamiltonian. A two-step variational procedure is used to obtain rotationally excited states of the system. The results of test calculations H2S and D2S with J = 0, 1, 5, and 10 are discussed along with computer-usage characteristics.
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  • 60
    ISSN: 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: Compact orbital GTO basis sets optimized with a nonrelativistic Hamiltonian, then decontracted, are utilized in an SCF treatment with a quasi-relativistic scalar Hamiltonian including the mass-velocity and one-electron Darwin operators. Ionization and excitation energies, orbital energies, and radial mean values obtained from different expansion patterns have been tested in atomic calculations for Ag and generalized for Cu and Au atoms. The one-electron spin-orbit operator is also used in an SCF treatment. Spin-orbit coupling energies are calculated for Cu, Ag, and Au atoms.
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  • 61
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio molecular quantum mechanics has been applied to a number of coupled ring systems including biterahedryl and cubylcubane. Basis sets at least as large as double zeta augmented by carbon d functions (DZ+d) were used throughout. For biterahedryl, electron correlation effects qualitatively alter the molecular structure, decreasing the central C—C bond distance while increasing the adjacent C—C distance. Carbonyl substituents, contrary to some previous thinking, do not qualitatively alter the bitetrahedryl structure. This finding points to a structural problem with the monomer bicyclo[3.1.0.02,6]hexane that carries over to the coupled bicyclo[3.1.0.02,6]hexyl molecule. Finally, for the cubylcubane molecule synthesized in 1988, the only significant difference between Hartree-Fock theory and experiment occurs for the central C—C distance, which, as in the other coupled molecules, is too long. These results significantly lengthen the (still very short) list of closed-shell hydrocarbon molecules for which Hartree-Fock theory encounters structural difficulties.
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  • 62
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optimization of the Mn-Mn distance in Mn2(CO)10 with various basis sets of at least doublezeta quality results in Mn-Mn bond lengths in the range of 3.07-3.15 Å, 0.2-0.25 Å longer than the experimental value of 2.895 Å. Incrementing the basis set with diffuse p functions (exponent 0.0332) on the carbon atoms improves the calculated bond length to a value of 2.876 Å at the CI level, as a consequence of a charge transfer between each Mn atom and the equatorial carbonyls of the other Mn atom. For Mn2(CO)9 four structures have been studied at the SCF and CI levels with assumed geometries. The structure with a symmetric bridging carbonyl turns to be much higher in energy at the SCF level. The two structures which are purely metal-metal bonded [corresponding to the departure of an axial or equatorial carbonyl from Mn2(CO)10] are nearly degenerate in energy and more stable than the structure with a semibridging carbonyl by 5 kcal/mol at the SCF level and 10-11 kcal/mol at the CI level (seemingly at variance with the conclusions of matrix experiments that favor the semibridging structure).
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  • 63
    ISSN: 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 band structure results of polypyrrole (PPY) and polythiophene (PTP) obtained with a double-zeta basis set are reported. The electronic density of states (DOS) of the various quasione-dimensional copolymers (superlattices) of the type (AmBn)x (A = pyrrole; B = thiophene) have been calculated numerically within the ab initio SCF tight-binding approximation. These copolymers on the basis of the band positions of PPY and PTP are found to belong to the class of type II (staggered) superlattices. The trends in their electronic properties as a function of (i) composition (m/n), (ii) block sizes m and n, and (iii) arrangement of blocks in the copolymer chain are discussed.
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  • 64
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    International Journal of Quantum Chemistry 42 (1992), S. 1025-1035 
    ISSN: 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 great deal of effort in recent years has been put into the development of a new band theory technique based on multiple-scattering theory. Some of the highlights of this development are described.
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  • 65
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An ab initio method for calculating the longitudinal linear polarizability of polymeric chains is described. This method is equivalent to an uncoupled Hartree-Fock scheme. It is applied to polyethylene and polysilane in minimal STO-3G and extended 4-31G basis sets. The study describes important techniques for solving the difficulties met in actual calculations: band reordering of the band structures, calculation of analytical derivatives of the energy bands ∊n(k) and LCAO coefficients cnp(k), and errors caused by the improper lattice sum truncations of the Hartree-Fock matrix.
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  • 66
    ISSN: 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 mathematical construction is presented that uniquely defines a set of spin-independent effective valence-shell Hamiltonian (Hv) three-body matrix elements. These spin-independent Hv matrix elements separate direct and exchange portions of the three-body Hv matrix elements and therefore provide the most natural form for comparisons with parameterization schemes of semiempirical electronic structure methods in which the three-body matrix elements are incorporated into semiempirical one- and two-body Hamiltonian matrix elements in an averaged manner. Ab initio Hv three-body matrix elements of O2 are computed through third order of quasidegenerate perturbation theory and are analyzed as a function of internuclear distance and atomic orbital overlap to aid in understanding how these three-body matrix elements may be averaged into semiempirical one- and two-body matrix elements. © 1992 John Wiley & Sons, Inc.
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  • 67
    ISSN: 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: Stretching force constants for formamide and its seven associated species involving two to four molecules hydrogen-bonded through linear and cyclic configurations and 10 structures containing formamide hydrogen-bonded with one to five water molecules are reported. Since ab initio calculations are rather inconvenient to perform on such big clusters and are time-consuming, CNINDO MO calculations were carried out using the gradient method. The results demonstrate, on the one hand, the feasibility of semiempirical calculations for the evaluation of trends in force constants for big clusters where generally ab initio calculations become much involved and, on the other hand, explain the effect of hydrogen bonding and cooperativity on force constants and vibrational spectra of biologically important systems composed of formamide in the condensed phase and its aqueous solutions. The C=O and N—H stretching force constants are found to reduce significantly on hydrogen bonding. The reduction in force constant is further enhanced when two cyclic dimers become associated through a linear hydrogen bond. The results indicate justification for the stabilization of the formamide structure with two cyclic dimers hydrogen-bonded together. The reduction in the force constants on hydrogen bonding also reflect the cooperativity contribution. The C=O and C—N stretching force constants for the structures corresponding to formamide in liquid and aqueous solution phases are in agreement with the experimental vibrational frequencies reported.
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  • 68
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The most efficient optimization methods implemented in the semiempirical package AMPAC are presented. They concern the minimization of the energy or of the gradient norm by either pseudo-Newton or quadratic procedures, the search for transition states, and the intrinsic reaction coordinate in conjunction with variational transition-state theories. Nonlocal methods such as simulated annealing are also introduced. © 1992 John Wiley & Sons, Inc.
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