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  • 1990-1994  (2,366)
  • Computational Chemistry and Molecular Modeling  (2,365)
  • Nuclear reactions
  • 101
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 21-42 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The present work is dedicated to the introduction of a new single-center expansion method derived from the Barnett and Coulson approach leading to reliable numerical algorithms. The basic concept lies in the use of the integral representations of the modified Bessel functions Iλ+1/2 and Kλ+1/2, rather than employing their closed analytical expressions that are known to involve large numbers that usually make the numerical calculation worthless. It is applied to two-center overlap and Coulomb integrals as well as to three-center nuclear attraction integrals useful in density functional theory. © 1994 John Wiley & Sons, Inc.
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  • 102
    Electronic Resource
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 103
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994), S. 225-240 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of basis sets and configuration interaction (CI) wave functions, both of which were constructed so as to systematically approach to the complete set limit and the full CI limit, were used for the ground state of Ne. These calculations yielded an estimated correlation energy of -0.3891 au, which is 99.6% of a recent theoretical estimate of -0.3905 au. The CI value, -0.3821 au, was obtained by SDCI calculation with seven reference configurations by using Slater-type orbitals (STOs) from s to h functions. © 1994 John Wiley & Sons, Inc.
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  • 104
    Electronic Resource
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994), S. 241-254 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: High-directional Monte Carlo (HDMC) simulations are performed to investigate the behavior of water molecules in the catalytic sites of human ras p21 proteins and the results are compared among three types of hydrated ras p21 proteins: the normal ras complexed with GDP (normal-p21/GDP), the mutant ras complexed with GDP (mutant-p21/GDP), and the normal ras complexed with GTP (normal-p21/GTP). Also, the original Metropolis Monte Carlo (MMC) simulation is performed on the normal-p21/GDP system. Comparison between HDMC and MMC shows the improved efficeincy of HDMC on this complicated system. Interaction between water molecules is represented by the TIP3P potential function. The solute-water interaction is represented by the Coulomb and van der Waals interactions from the CHARMM force field spliced with TIP3P. The mean-square displacements and dipole correlation functions are reported. We specifically examined the hydrogen-bond interactions existing near the catalytic sites, by the distribution of small water clusters connected by the hydrogen bond and by the average lengths and fluctations of hydrogen bonds between water molecules and solute atoms. Water molecules form different local structures in different types of the p21/GDP(GTP) complexes. The structure and motion of water molecules near the catalytic sites of the proteins are discussed in detail. © 1994 John Wiley & Sons, Inc.
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  • 105
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994), S. 265-283 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Feynman path integral method is applied to the many-electron problem of quantum chemistry. We begin with investigating the partition function of the system in question; then, “a classical path of electron” that corresponds to the Hartree-Fock approximation is obtained by minimizing the thermodynamic potential of the system with respect to the electron coordinate. The next-order approximation is obtained by evaluating the deviation from this classical path, which is approximately written by an easily integrable Gaussian integral. The result is expected to be the random-phase approximation. As numerical examples, the hydrogen molecule and butadiene are treated. © 1994 John Wiley & Sons, Inc.
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  • 106
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994), S. 313-318 
    ISSN: 0020-7608
    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 (TZV*, SBK*, and 3-21G* or 6-31G* basis sets) calculations were performed to predict the geometries and gas-phase proton affinities of Li2O, LiOH, LiNH2, Na2O, NaOH, NaNH2, K2O, KOH, and KNH2. © 1994 John Wiley & Sons, Inc.
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  • 107
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994), S. 329-333 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ground state of the 2D electron crystal is investigated using a localized representation for the electrons. Assuming a neutralizing background, calculation for a nonmagnetic electron crystal is performed. The areal electron densities corresponding to the crystallization for different rs values are computed and these are compared with the available experimental areal densities. An upper limit for the obtainable areal density has been predicted from our results. © 1994 John Wiley & Sons, Inc.
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  • 108
    Electronic Resource
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994) 
    ISSN: 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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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994), S. 343-344 
    ISSN: 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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  • 110
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994), S. 369-388 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The general theoretical background for solving approximately the contracted Schrödinger equation (CSchE) in a self-consistent (SC) way has recently been proposed [F. Colmenero and C. Valdemoro, Phys. Rev. A 47, 979 (1993)]. Here, a spin-free procedure is developed and the convergence of the SC iterative process is analyzed using as test cases the beryllium atom and four isoelectronic ions in their ground states. Damping and extrapolation procedures are employed to improve and accelerate the convergence. The results obtained are in very close agreement with those obtained by means of the full configuration-interaction (FCI) method. © 1994 John Wiley & Sons, Inc.
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  • 111
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    International Journal of Quantum Chemistry 51 (1994), S. 389-396 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Self-consistent screening arises in all many-body problems where one-particle equations containing functionals of the solutions are considered. In the theory of simple metals, e.g., these one-particle equations are usually solved to second-order perturbation in the crystal pseudopotential and to first order in the screening. However, higher-order screening terms have a sizable effect on the form factors, the electron charge density, and the screened pseudopotential. The effects of second order are presented for the bcc simple metals Na and Ba. A nonlocal exchange-correlation potential has been used since local density approximations such as ρ1/3 cause difficulties in the electron-electron dielectric function. © 1994 John Wiley & Sons, Inc.
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  • 112
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    International Journal of Quantum Chemistry 51 (1994), S. 425-437 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rapidly developing field of statistical theory of spectra of many-electron systems is briefly reviewed. In particular, new formal developments, their implementations in studying general properties of the model spaces, and links to the reduction problem are addressed. Applications in molecular and atomic spectroscopy are also discussed. © 1994 John Wiley & Sons, Inc.
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  • 113
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    International Journal of Quantum Chemistry 51 (1994), S. 439-445 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The basic idea of the deformable or soft-body model of a molecule, which starts from a new definition of the reference configuration, is presented. It is shown that this model introduces into the Hamiltonian a new part that describes deformation energy of a molecule brought about by its rotation. This part leads to nonlinear terms, whose numerical values were compared with the semiempirical approach. © 1994 John Wiley & Sons, Inc.
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  • 114
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    International Journal of Quantum Chemistry 51 (1994), S. 465-472 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The thermal rate constant of a bimolecular reaction is shown to be expressible as the time limit of the time integral of a correlation function involving the reactive flux at two different points along the path leading from reactants to products. This extension provides some additional flexibility in the finite basis-set representation of the rate given by Miller-Schwartz-Tromp [J. Chem. Phys. 79, 4889 (1983)]. Some of the features of this expression are examined in the case of one-dimensional Eckart barrier. © 1994 John Wiley & Sons, Inc.
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  • 115
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    International Journal of Quantum Chemistry 51 (1994), S. 487-497 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The first two moments of the spectral density distributions of the second- and the third-order spin-adapted reduced Hamiltonian (SRH) matrices have been calculated and compared with the corresponding quantities describing the full configuration interaction (FCI) Hamiltonian matrix. It has been demonstrated that the differences between the SRH and FCI spectral density distributions sharply increase with increasing difference between the number of electrons and the order of SRH. © 1994 John Wiley & Sons, Inc.
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  • 116
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of physical processes, such as autoionization, predissociation, ac- or dc-field-induced ionization, multiphoton dissociation, or chemical transformations, can be formulated as problems involving a nonstationary state satisfying a time-independent complex eigenvalue Schrödinger equation (CESE). The CESE gives rise to all the conceptual and practical difficulties associated with the polyelectronic structures of excited states, as well as novel ones due to the presence of external fields and to the physical significance of the continuous spectrum. In a series of articles from this institute, it has been shown how advanced electronic structure theory and methods suitable for excited states can be integrated in a practical way into selected elements of the rigorous theory of discrete states interacting with the continous spectrum in order to solve the CESE nonperturbatively and efficiently and compute properties such as positions and widths of inner hole or multiply excited states, multiphoton ionization rates, multichannel predissociation lifetimes, nonlinear static and frequency-dependent polarizabilities, and tunneling rates. The present article constitutes a review of the basic features of this theory and its computational methods. © 1994 John Wiley & Sons, Inc.
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  • 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 microscopic formulation of solution chemical reactions, taking reactants and medium structures into consideration, is presented on the basis of microscopic understandings obtained by recent quantum chemical methods (i.e., ab initio molecular orbital theory, etc.). Assuming thermal equilibrium of the medium bath, an effective internal Hamiltonian is derived, and, further, its derivative with respect to internal normal coordinates is proved explicitly to give the same force field as is provided by the free-energy surface or potential of mean force. The free-energy surface can be expressed in the composite normal coordinate system (CNCS) consisting of some normal coordinate systems of isolated reactants and surrounding solvent molecules (i.e., medium solvent molecules). In CNCS, in use of diagonal elements obtained in the Hessian matrix of the free-energy surface, effective normal-mode frequencies, which reflect the equilibrium solvent effect, are estimated. Furthermore, on the generalized Langevin equation (GLE) treatment, a closed expression of the time-dependent frictional coefficient is derived on a microscopic basis, reflecting the reactant and solvent structures. The nonequilibrium effect is estimated by an analytical expression similar to that in the Grote-Hynes theory. The rate constant is evaluated for a typical model system and it is shown that the equilibrium rate constants should be reduced by a factor 0.997. Finally, it is concluded that the present microscopic theory is reasonably applicable to the estimation of chemical reaction rate constants in solution. © 1994 John Wiley & Sons, Inc.
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  • 118
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    International Journal of Quantum Chemistry 51 (1994), S. 211-224 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The first nonrelativistic lower bound to the ground state of the lithium atom is give with E0 〉 -7.47816 au using the method of variance minimization and an extension of Temple's formula. With large Hylleraas-CI basis sets, high-precision upper bounds and isotope shifts are calculated for the three lowest 2S states of the lithium atom, which are best to date. © 1994 John Wiley & Sons, Inc.
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  • 119
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994) 
    ISSN: 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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  • 120
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    International Journal of Quantum Chemistry 51 (1994), S. 255-264 
    ISSN: 0020-7608
    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, new technique for the evaluation of the similarity of molecular shapes is presented. The concept of semisimilarity (asymmetric similarity) is applied within the topological-geometrical framework of scaling-nesting similarity measures of molecules. The practical application of these similarity measures is illustrated by the examples of the three-dimensional formal bodies of electronic charge densities of a set of simple molecules. © 1994 John Wiley & Sons, Inc.
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  • 121
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    International Journal of Quantum Chemistry 51 (1994), S. 285-291 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: To accelerate the convergence of the HH expansion, we modified the HH-GLF method, a new simple hyperspherical harmonic method proposed recently by us, into the CFHH-GLF method. Applications of the CFHH-GLF method to the three-body systems He and e- e- e+ exhibit very fast convergence with number of HH basis sets. With only 36 HH and five GLF, we obtain the ground-state energy of -2.90371 au for He, compared with the exact value of -2.90372 au, and with only 36 HH and 10 GLF, we obtained the ground-state energy of -0.26188 au for e- e- e+, compared with the exact value of -0.26200 au. We formulate the CFHH-GLF method in this article. © 1994 John Wiley & Sons, Inc.
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  • 122
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    International Journal of Quantum Chemistry 51 (1994), S. 293-305 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Fourier grid Hamiltonian method is used to calculate the response properties of different types of 1-d (one-dimensional) quantum oscillators in a uniform static electric field. The calculations are potentially exact. Excepting the harmonic oscillator, the other model oscillators studied are seen to possess nonlinear polarizabilities. In general, the polarizabilities are not monotonic functions of appropriate vibrational quantum numbers. The exact nature of this vibrational-state dependence of polarizabilities is shown to depend on the type of mechanical anharmonicity in which the nuclei move and the nature of electrical anharmonicity characterizing the field-oscillator coupling. The large vibrational contribution to nonlinear polarizabilities often predicted for real diatomics could therefore originate from the mechanical and electrical anharmonicities of the potential in which the nuclei move when placed in a static electric field. © 1994 John Wiley & Sons, Inc.
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  • 123
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    International Journal of Quantum Chemistry 51 (1994), S. 307-312 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The multiconfiguration time-dependent Hartree-Fock (MC TDHF) approximation is applied to study the excited states of the BH molecule. The potential energy curves and spectroscopic properties of the excited states are analyzed. The MC TDHF results are compared to similar MC SCF calculations for individual excited states. © 1994 John Wiley & Sons, Inc.
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  • 124
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    International Journal of Quantum Chemistry 51 (1994), S. 319-328 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The MS-LSD method remains a method of interest when rapidity and small computer resources are required; its main drawback is some lack of accuracy, mainly due to the muffin-tin distribution of the potential. In the case of large clusters or molecules, the use of an empty sphere to fill, in part, the large intersphere region can improve greatly the results. Calculations bearing on C60 has been undertaken to underline this trend, because, on the one hand, the fullerenes exhibit a remarkable possibility to fit a large empty sphere in the center of the cluster and, on the other hand, numerous accurate calculations have already been published, allowing quantitative comparison with our results. Our calculations suggest that in case of added empty sphere the results compare well with the results of more accurate calculations. The calculated electron affinity for C60 and C60- are in reasonable agreement with experimental values, but the stability of C602- in gas phase is not found. © 1994 John Wiley & Sons, Inc.
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  • 125
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    International Journal of Quantum Chemistry 51 (1994), S. 335-341 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electrostatic potential and electric field maps of some suosan series of sweeteners have been studied. Geometries optimized using the MNDO molecular orbital method and charge distributions obtained by the Mulliken population analysis were employed in the calculations. Sweetness potencies of the molecules were compared with electrostatic potentials and electric field strengths near the terminal COOH group. The observed sweetness potencies of the compounds and calculated electric fields near the carbonyl oxygen of the COOH group are found to be strongly correlated. © 1994 John Wiley & Sons, Inc.
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  • 126
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    International Journal of Quantum Chemistry 51 (1994), S. 345-356 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Among the methods that may be used to carry out multicenter integrals over STOS, the one-center two-range expansion is probably the oldest one. However, the convergence of the leading series is generally slow and, hence, time-consuming. Therefore, it was the aim of the present work to show that the so-called nonlinear sequence transformations may be of substantial help for improving the rate of convergence of such series. © 1994 John Wiley & Sons, Inc.
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  • 127
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The use of both the Laplace transformation technique and the finite element method has been analyzed in an electrochemical system. The diffusion-controlled adsorption process of a species onto an electrode surface is studied in detail. © 1994 John Wiley & Sons, Inc.
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  • 128
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    International Journal of Quantum Chemistry 51 (1994), S. 397-405 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Slater-type orbitals (STOs) with a single-exponent by shell or by subshell have been constructed to reduce the number of integrals evaluated in the electronic calculations. The expansion of orbitals in these new basis sets has been carried out in detail for the ground state of the Ne atom. We have carried out a study of STO basis sets with a different size for this atom that could help to propose empirical rules for the selection of these basis sets for other atoms. The usefulness of STOs with single-exponent by shell and subshell and the splitting of s and p functions are discussed. © 1994 John Wiley & Sons, Inc.
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  • 129
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    International Journal of Quantum Chemistry 51 (1994), S. 417-423 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The strategy of using expansions in spherical harmonics of displaced Slater-type orbitals for the evaluation of multicenter molecular integrals is reviewed and projected. Our Löwdin α-function method is augmented by computer algebra and C, E, and F matrices with rational elements. This permits exact evaluations of each term in an infinite sum using a commercial computer algebra program to investigate the three-center nuclear attraction integral. Each term (harmonic potential) is important for proposed use in numerical evaluations of multicenter integrals. © 1994 John Wiley & Sons, Inc.
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  • 130
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    International Journal of Quantum Chemistry 52 (1994), S. 457-463 
    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: Although several authors proposed the existence of long-range correlations in DNA sequences [W. Li and K. Kaneko, Europhys. Lett. 17, 655 (1992); R.F. Voss, Phys. Rev. Lett. 68, 3805 (1992); C.-K. Peng et al., Nature 356, 168 (1992)], we claim that the real character of the correlations remains uncertain. To corroborate long-range correlations, one should prove that the correlation functions decay as an inverse power of the distance between the nucleotides in the DNA sequence. Instead of a direct calculation of the correlation functions, one can calculate either the Fourier transforms of the sequences or the characteristics of “DNA walk.” In our previous work we pointed out that the character of the results depends upon the way the DNA sequences are chosen. When looking for the correlations separately in introns and separately in exons, we found that the resulting correlations are short range. In this work, we discuss the numerical aspects of two computational approaches: the calculation of correlation function and the evaluation of “DNA walk” characteristics. We present the analytic results for the case of exponential correlations. © 1994 John Wiley & Sons, Inc.
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  • 131
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    International Journal of Quantum Chemistry 52 (1994), S. 491-506 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quantum chemical calculations for two different kinds of native proteins (pig insulin and hen egg white lysozyme) were done by the extended negative factor counting method in which the matrix elements have been calculated at the ab initio level with the help of a minimal basis and the simulation of the aqueous solution environment. The hopping conductivities were worked out by the formulas of the random walk theory of Lax and co-workers. The electronic density of states of these native proteins confirmed the conclusions obtained previously from aperiodic model peptides chains. The results show that the ac conductivity vs. frequency curve of these native proteins lies in the range of some typical good inorganic amorphous conductors and thus confirm that proteins, if doped, are amorphous conductors. The behavior of the ac conductivities of the proteins in different ranges of frequencies are discussed. © 1994 John Wiley & Sons, Inc.
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  • 132
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    International Journal of Quantum Chemistry 52 (1994), S. 563-574 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The formalism of strictly localized geminals (SLGs) is summarized. It is shown that the SLG wave function serves as an appropriate multiconfigurational reference state that can easily be improved by perturbational, CI- or coupled cluster-type procedures. The possibility of expanding the geminals in the basis set of localized Hartree-Fock molecular orbitals (LMOs) is discussed. Sample calculations on H4, CH4, H2O, and He…He systems are reported. © 1994 John Wiley & Sons, Inc.
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  • 133
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    International Journal of Quantum Chemistry 52 (1994), S. 593-607 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three programs capable of computing algebraic formulas for Wigner 3-j, 6-j, and 9-j symbols have been written in the Maple* program language. These programs can also compute numerically exact values of 3-j, 6-j, and 9-j symbols. © 1994 John Wiley & Sons, Inc.
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  • 134
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    International Journal of Quantum Chemistry 52 (1994), S. 617-627 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We propose a model for the potential of thin metal films, in which (1) we reproduce self-consistent effective potentials (i.e., Lang and Kohn), in fact, better than any approximation in the existing literature, (2) we parameterize the potential as a function of the film thickness, and (3) we approach the classical image potential for large separation from the surfaces. Our potential does not diverge as the electron approaches the surfaces and takes into account the effect of the two surfaces. It reproduces the first peak in the effective potential, which is a Friedel oscillation. We present a simple dependence of the potential with the film thickness that (1) allows a direct description of the quantum size effects and (2) shows how the bulk surface potential is reached. This makes our model useful in the interpretation of tunnel experiments, field emission, and scanning tunneling microscopy. © 1994 John Wiley & Sons, Inc.
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  • 135
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    International Journal of Quantum Chemistry 52 (1994), S. 117-125 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The local mechanisms of dissociative chemisorption and reactive etching are investigated by molecular models modeling ≡SiOH groups and ≡SiOSi≡ bridges at SiO2 surfaces. The elementary reaction paths for the reactions of HF and HCl with Si—O bonds of the models, Si(OH)4 and (HO)3SiOSi(OH)3, are calculated by the Hartree-Fock method. The local transition-state structures are nearly plane quadrangles with neighboring pentacoordinated silicon and tricoordinated oxygen atoms. For the reaction and activation energies, the electron correlation is considered by the Møller-Plesset perturbation theory. The results agree with experimental findings. © 1994 John Wiley & Sons, Inc.
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  • 136
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    International Journal of Quantum Chemistry 52 (1994), S. 135-146 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The so-called shake-up satellites accompanying C1s photoionization in C60 have been studied theoretically by means of INDO/CI. It is found that the lowest shake-up satellites in C60 correspond to global charge-transfer excitions that move charge from the opposite side of the fullerence cage to the core-hole region, whereas higher-energy satellites tend to move charge from regions nearer to the core hole. Analogies are drawn, with, on the one hand, smaller model molecules such as napthalene and acenaphtylene and, on the other hand, infinite systems such as graphite. © 1994 John Wiley & Sons, Inc.
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  • 137
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    International Journal of Quantum Chemistry 52 (1994), S. 153-163 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The vibration and rotation of molecules affects nuclear spin-spin coupling constants. This manifests itself as a temperature dependence of the coupling and also as an isotope effect (after allowing, where necessary, for differing magnetogyric ratios of the two nuclei involved in the isotopic substitution). Within the Born-Oppenheimer approximation, a nuclear spin-spin coupling surface can be defined for each pair of coupled nuclei. This surface is sampled by the nuclei as they undergo the excursions about equilibrium geometry that are governed by the force field. An accurate ab initio carbon-proton spin-spin coupling surface for the methane molecule has been calculated. This was obtained by summing the surfaces for each of the four contributions - Fermi contact, spin-dipolar, orbital paramagnetic, and orbital diamagnetic - expressed as power series in terms of symmetry coordinates. Preliminary calculations for 13CH4 and 13CD4 give a difference of only 6% between the calculated and observed nuclear motion contributions. The observed temperature dependence is also accounted for by the calculations. For these isotopomers, bond stretching plays the dominant role. © 1994 John Wiley & Sons, Inc.
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  • 138
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    International Journal of Quantum Chemistry 52 (1994), S. 507-514 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Application of a purely algebraic least-squares approach in a spline basis to the determination of continuum orbitals in the single-electron molecular case illustrates the excellent numerical properties of the method. Initial work based on a single-center expansion is extended to an LCAO formulation within a single-particle Hamiltonian and to the full multielectron problem in the one-center scheme. Preliminary results on H2+ and He photoionization are reported. © 1994 John Wiley & Sons, Inc.
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  • 139
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    International Journal of Quantum Chemistry 52 (1994), S. 549-562 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ability of the QDPT CI method, in which the quasi-degenerate perturbation theory is applied within the configuration interaction (CI) approach, in dealing with the calculation of excitation and photoionization spectra is shown through an overview of comparisons between the QDPT CI values and the full CI ones in the same basis set. A direct comparison with the experimental data is given for the core photoelectron spectrum of Ne and the valence photoelectron spectra of Ar and HCl. The quality of the results obtained is very satisfactory, both as regards the energies and the wave functions, indicating the validity and the flexibility of the method that can confidently be applied also in cases where strong correlation effects are present. © 1994 John Wiley & Sons, Inc.
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  • 140
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    International Journal of Quantum Chemistry 52 (1994), S. 609-616 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We discuss here a new formal way to evaluate the electronic correlation energy of a molecular system. We use the path integral formalism in the regime of the molecular orbitals. The semiclassical approximation in this procedure is an expansion of the effective action Seff around the classical closed orbits into generalized phase space keeping only the quadratic fluctuation. Thus, after some transformations, we apply to the final expression analogous conditions to the Bohr-Sommerfeld quantification rules. © 1994 John Wiley & Sons, Inc.
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  • 141
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    International Journal of Quantum Chemistry 52 (1994), S. 651-655 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This theoretical study was to determine the relative stability of the different species proposed experimentally and to establish which of the two pathways suggested is the most probable. A conformational study of the geometries of the stable compounds has been carried out using the semiempirical quantum mechanic method AM1. © 1994 John Wiley & Sons, Inc.
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  • 142
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    International Journal of Quantum Chemistry 52 (1994), S. 673-680 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The equilibrium geometry and UV-visible spectra of C70 were examined using semiempirical INDO/2 and INDO/CI methods. The results obtained are in good accord with experimental results. On the basis of correct electronic spectra, calculations of the nonlinear third-order optical susceptibility (γijkl) of C70 were performed using the INDO/SDCI method combined with a sum-over-states expression. The calculated value for 〈γ〉 (-2 ω, ω, ω, O) is 0.882 × 10-33 esu (ω = 1.91 μm), which is in good agreement with observation. © 1994 John Wiley & Sons, Inc.
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  • 143
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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 computational studies to understand the interaction of polycyclic aromatic hydrocarbons (PAHs) with biomolecular systems, the semiempirical method AM1 has been used previously to determine the geometry of the PAH and its metabolites and relevant intermediates. A number of studies have shown that AM1 provides geometries for parent PAHs that are acceptably close to experimentally determined structures. However, many of the properties that determine the manner by which PAHs interact with biological nucleophiles depend on the structure of metabolites and reactive intermediates where less experimental information is available. In a previous study, we used AM1 to obtain the molecular geometries of reactive intermediates of cyclopenta-PAHs (cPAHs) and then used single-point Hartree-Fock calculations, with the gaussian 3-21g basis set, to obtain molecular energies and charge distributions, in order to predict the direction of epoxide ring opening. Recent advances in the availability of computational hardware and software have provided other, more rigorous, methods for approaching this problem. In this study, we used hartree-fock methods in the gaussian series of programs employing the 3-21g and 6-31g basis sets and the local density functional method Dmol to obtain molecular geometries, energies, and charge distributions of the epoxides and the two potential hydroxycarbocations that could result from protonated ring opening, for a series of cPAHs. We have also performed the same calculations with AMSOL/SM2, a semiempirical method that adds the effect of the aqueous environment to the AM1 Hamiltonian. The division of the cPAHs into classes is not altered by these more rigorous calculations. The inclusion of water in the Hamiltonian has a greater effect on the results than using the ab initio methods to obtain the structure. © 1994 John Wiley & Sons, Inc.
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  • 144
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    International Journal of Quantum Chemistry 52 (1994), S. 829-835 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Experimental results on 3d Oh complexes in insulators reveal that 10Dq α R-n, where R is the metal-ligand distance and n is close to five. This strong dependence determines the Huang-Rhys factor, S(A1g), associated to the symmetric A1g mode of the first excited state of complexes like MnX64- and CrX63- (X = halide) and makes it possible to measure R changes down to ∽ 10-3 Å. This work is devoted to understanding, within a molecular orbital framework, the microscopic origin of such a dependence, which is related to the corresponding one displayed by the transferred spin densities fσ, fs, and fπ. The analysis is focused on MnF64-. As a main result, it is shown that though fσ ≫ fs the interaction between d(eg) orbitals and 2s orbitals of F- is not only primarily responsible for the R dependence of 10Dq but also makes a significant contribution to the 10Dq value itself. The present work thus shows that the significant dependence of 10Dq upon R is ultimately related to the strong dependence of fs and the isotropic superhyperfine constant As upon R displayed by the experimental results of several 3d impurities. © John Wiley & Sons, Inc.
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  • 145
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    International Journal of Quantum Chemistry 52 (1994), S. 867-877 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Coordination compounds are usually symmetrical molecules with degenerate orbitals. Hence, the individual multiplet states arising from open-shell configurations can, in general, not be expressed by a single determinant. We have therefore exploited symmetry to the largest possible extent in order to simplify the relation between the multiplet splitting and single-determinant energies and thus developed a new method based on vector coupling to keep the computational effort to a minimum. A system of computer programs working on both mainframe and personal computers has been developed, carrying out for any desired point group the required group theoretical manipulations. The description of the method is illustrated by considering three practical examples. © 1994 John Wiley & Sons, Inc.
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  • 146
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    International Journal of Quantum Chemistry 52 (1994), S. 919-926 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Within the framework of density functional theory (DFT), we calculate the ground-state electronic properties of CdxSn1-x Te using norm-conserving pseudopotentials in connection with the local density (LDA) and virtual crystal approximation (VCA). Our particular interest is in the influence of the Cd-4d and the Sn-4d electrons by comparing results obtained with pseudopotentials, which either consider the d-electrons explicitly or in the frozen core. In the mixed crystal system CdxSn1-x Te, the transition from a ten- (x = 0) to an eight-electron (x = 1) system is realized, which is accompanied by a change of the crystal structure from rock salt (SnTe) to zinc blende (CdTe). By calculating the ground-state energies, we find the equilibrium lattice constant as the function of x and the bulk modulus, as well as the crossover value of x for the transition from rock-salt to zinc blende. The calculated lattice constants and bulk moduli are in much closer agreement with experimental data if the d-electrons are taken into account explicitly, whereas the crossover is rather insensitive with respect to the d-electrons. In view of the electronic charge density, we demonstrate the decrease of ionicity for increasing x. © 1994 John Wiley & Sons, Inc.
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  • 147
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    International Journal of Quantum Chemistry 52 (1994), S. 963-971 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structures, harmonic force fields, and hyperfine coupling constants of a number of representative σ radicals have been investigated by the density functional approach including nonlocal corrections. Scaling of CH bond lengths and stretching constants leads to accurate structures and force fields. Hyperfine coupling constants reach, at least, the level of the most sophisticated conventional post-Hartree-Fock models. This suggests that the density functional approach is a promising theoretical tool for the study of relationships between structure and spectroscopic properties of large free radicals. © 1994 John Wiley & Sons, Inc.
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  • 148
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    International Journal of Quantum Chemistry 52 (1994), S. 1011-1015 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: LCGTO-LSD and LCGTO-NLSD methods have been tested for the study of water-carbon dioxide weakly bound binary complex. Different local and nonlocal exchange-correlation energy functionals and many grid radial points have been used. Results show that both nonlocal corrections and a large number of radial points in the grid are mandatory for well reproducing the experimental data. © 1994 John Wiley & Sons, Inc.
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  • 149
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    International Journal of Quantum Chemistry 52 (1994), S. 1039-1049 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Local and nonlocal density functional computations have been carried out to study the electronic structure and the equilibrium geometry of the isoelectronic series Cr(NC)4, Mn(NO)3(CO), Fe(NO)2(CO)2, and Co(NO)(CO)3 and model compounds Fe(NO)2L2 (L = Cl, HCN, NH3, PH3, and C2H4). The structure of Fe(NO)2(C4H6) is also described. The discussion is focused on structural modifications through a change of ligand, in particular those concerning the metal-nitrosyl conformation (linear vs. bent). Though this is a preliminary study of metal-nitrosyl properties by DFT methods and more computations are required to analyze the mechanism of homogeneous catalysis processes, our results support the hypothesis that structural reorganization from linear to bent metal-nitrosyl plays a key role in some reactions, such as in butadiene dimerization. © 1994 John Wiley & Sons, Inc.
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  • 150
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    International Journal of Quantum Chemistry 52 (1994), S. 1027-1038 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An extensive study of the application of a large set of correlation energy functionals to atomic systems is presented. Hartree-Fock densities and, sometimes, generalized-valence-bond wave functions are used. The functionals have been grouped into four sets: (I) local density and local spin density, (II) local spin density with self-interactioon correction, (III) local spin density with gradient correction, and (IV) methods found on the correlation factor approach. In this bench mark, we report the results of calculations made for the ground states of the first and second rows, their anions and cations, and the two- and four-isoelectronic series. © 1994 John Wiley & Sons, Inc.
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  • 151
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    International Journal of Quantum Chemistry 52 (1994), S. 1081-1111 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The geometrical structures and properties of small cationic boron clusters Bn+(n = 2-14) have been investigated using the local spin density (LSD) formalism. The nonlocal correction has also been calculated. The linear search for minima on the potential energy surface has been performed using analytical gradients of the LSD total energy. Most of the final structures of the cationic boron clusters prefer planar or quasi-planar nuclear arrangements and can be considered as fragments of a planar surface or as segments of a sphere. The calculated adiabatic ionization potentials of Bn exhibit features that are analogous to those of measured ionization potentials for boron clusters. Most of the calculated normal modes of the cationic clusters have frequencies that are around 1000 cm-1 and have strong infrared intensities, and they correspond quite well with analogous properties of solid boron. © 1994 John Wiley & Sons, Inc.
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  • 152
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    International Journal of Quantum Chemistry 52 (1994), S. 1145-1151 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An analytical and computationally efficient formalism for the evaluation of rotationally averaged cross sections for X-ray and high-energy electron scattering from molecular systems is given. Its application for both SCF and CI wave functions is discussed. © 1994 John Wiley & Sons, Inc.
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  • 153
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    International Journal of Quantum Chemistry 52 (1994), S. 1177-1198 
    ISSN: 0020-7608
    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 formalism is presented for reliable calculations of the energetics of hole self-trapping in semi-ionic solids with mixed valence bands. Unlike previous model-Hamiltonian-type approaches, it is based on self-consistent quantum chemical INDO simulations of the atomistic and electronic structure of a self-trapped hole, making no a priori assumptions about a particular form of its localization (if any). This formalism is applied to the problem of hole self-trapping in corundum crystals (α-Al2O3). The hole self-trapping is found to be energetically favorable in the form of a diatomic O2 molecule with strong covalent bonding quite similar to the self-trapped hole (VK-center) in alkali halides. The so-called localization energy (i.e., the energy that is required to localize the Bloch-like wave packet of the free hole on the molecule, as the first stage of further trapping) is essentially less than one-half of the upper valence band width, which is the estimate commonly used for ionic solids. © 1994 John Wiley & Sons, Inc.
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  • 154
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    International Journal of Quantum Chemistry 52 (1994), S. 155-172 
    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 coupled cluster method is widely recognized nowadays as providing an ab initio method of great versatility, power, and accuracy for handling in a fully microscopic and systematic way the correlations between particles in quantum many-body systems. The number of successful applications made to date within both chemistry and physics is impressive. In this article we review our recent extensions of the method which now provide a unifying framework for also dealing with strongly interacting infinite quantum lattice systems described by a Hamiltonian. Such systems include both spin-lattice models (such as the anisotropic Heisenberg or XXZ model) exhibiting interesting magnetic properties, and electron lattice models (such as the tJ and Hubbard models), where the spins or fermions are localized on the sites of a regular lattice; as well as lattice gauge theories [such as the Abelian U(1) model of quantum electrodynamics and non-Abelian SU(n) models]. Illustrative results are given for both the XXZ spin lattice model and U(1) lattice gauge theory. © 1994 John Wiley & Sons, Inc.
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  • 155
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    International Journal of Quantum Chemistry 52 (1994), S. 181-194 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A computational study, using relativistic effective core potentials, is presented of transition metalmain group multiply bonded complexes, of interest in the context of catalysis and chemical vapor deposition of TM/MG materials. Model d0 transition metal complexes chosen are of the general form ClnME where M = Zr (n = 2), Ta (n = 3), and W (n = 4). Main group elements of interest are the tetrels (E = C, Si, Ge, Sn), pnictogens (E = N, P, As, Sb), and chalcogens (E = O, S, Se, Te). A comparison between calculated metric data and available experimental data for a wide range of TM = MG complexes will help in further assessing efficient computational approaches to TM complexes, particularly of the heavier MG elements, as a function of metal, ligand and level of theory. In the present work restricted Hartree Fock (RHF) and Møller-Plesset second order perturbation theory (MP2) wavefunctions were employed. In most cases there are small differences between RHF and MP2 calculated geometries, with both methods showing good agreement with experimental data, suggesting these approaches will be suitable for the study of larger, more experimentally relevant models. Changes in ZrE bond lengths for E = chalcogen (upon going from RHF to MP2) suggest a fundamentally different description between the Zr-oxo bond and heavier chalcogens, a result supported by recent experimental data for a series of Zr-chalcogenidos. To date no examples have been reported of arsinidene and stibinidene complexes. Computational results show similar behavior among the heavier pnictogen complexes, i.e., LnM = EH (E = P, As, Sb), suggesting that strategies used to synthesize phosphinidenes may be suitable in the search for the first LnM = AsR and LnM = SbR complexes. Additionally, calculations suggest that design of ligand sets which yield linearly coordinated phosphinidenes (and presumably As and Sb analogues) will lead to phosphinidenes with stronger metal-pnictogen bonds and increased thermodynamic stability versus nonlinearly coordinated examples. © 1994 John Wiley & Sons, Inc.
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  • 156
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    International Journal of Quantum Chemistry 52 (1994), S. 57-77 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Triglycerides, the major components of neutral lipids, are important biomaterials, as they take part in the edification of membranes. In this perspective, the consideration of biological membranes at a molecular level requires detailed knowledge of the preferred conformations of the triglycerides in their various polymorphic forms. In this context, we adapted a molecular modeling approach, which allows the simulation the three-dimensional structure of the different polymorphic forms (α, β′, and β) valid for any triglyceride. Their conformational analysis is based on molecular mechanics calculations, as follows: First, a large number of isolated molecular structures were generated in a systematic structuretree analysis. For their generation, atomic charges within the Mulliken scheme, calculated at the ab initio RHF-LCAO-MO-SCF level (6-31G), were considered. The lowest-energy conformers were, next, correlated with experimental data (NMR, powder X-ray diffraction) in order to select α, β′, or β structures. Then, in a second step, these selected conformers were assembled, in head-to-tail dimers in order to form a monolayer. For this step, the use of potential-derived atomic charges is known to be more suitable. In this study, we consider triglycerides derived from the predominant fatty acids, i.e., stearic, elaidic, and oleic acids. © 1994 John Wiley & Sons, Inc.
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  • 157
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Artemisinin and related molecules are potential antimalarials that contain the 1,2,4-trioxane ring system. Several new derivatives have been synthesized and tested in Geneva, and this article presents the results of a systematic study of the structure of these molecules, both by the semiempirical PM3 method and using ab initio SCF methods. The results highlight the feasibility of full optimizations with 3-21G and 6-31G* basis sets for these large molecules. Molecular electrostatic potential (MEP) maps are evaluated and used in an attempt to identify the key features of the molecules that are necessary for their activity. There is good agreement between the PM3 and ab initio maps as to the qualitative predictions. © 1994 John Wiley & Sons, Inc.
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  • 158
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    International Journal of Quantum Chemistry 52 (1994), S. 361-377 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The extreme sensitivity of the absorption spectrum to small changes in the medium polarity has made Reichardt's dyes useful molecular probes in the study of micelle/solution interfaces and phospholipid bilayers. This work reports preliminary results of semiempirical quantum chemical calculations on some conformations of 2,6-diphenyl-(2,4,6-triphenyl-1-pyridinium)-N-phenoxide betaine (Reichardt's betaine, RB), which exhibits negative solvatochromic effects. We have used the AM1 Hamiltonian of Dewar in the geometry optimizations, and the Intermediate Neglect of Differential Overlap method parameterized for spectroscopy (INDO/S). For RB, two low-lying conformations have been found. The small difference in energy between them suggests that both forms may be present in solution, an observation confirmed by calculations on the spectra using the SCRF model: the superposition of the calculated spectra for these two forms matches the experimental spectra very well. For nonpolar solvents, the general pattern consists of variation of Et(30) concurrent with variation of the dielectric constant. We have also carried out calculations for solvents which form specific (e.g., H-bond) binding to the solute, namely methanol and water, using a supermolecule approach. Our results are in excellent agreement with the experiment and present an accurate description of the spectra. © 1994 John Wiley & Sons, Inc.
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  • 159
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    International Journal of Quantum Chemistry 49 (1994), S. 11-19 
    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: The Möbius transformations recently proposed for integrating functions with a sharp peak close to a single boundary of integration (H.H.H. Homeier and E.O. Steinborn, J. Comput. Phys. 87, 61, 1990) are used to extend Ruedenberg's algorithm for two-center exchange integrals to cases with very diffuse STOs. © 1994 John Wiley & Sons, Inc.
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  • 160
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    International Journal of Quantum Chemistry 52 (1994), S. 469-480 
    ISSN: 0020-7608
    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 band structure calculations are carried out on extended regular polypropylene chains (isotactic helical, syndiotactic helical, and zigzag planar) to assess qualitatively the dependence of the energy band structure and density of states on the molecular geometry. It is found that discrimination between the two syndiotactic form should be possible from spectral features in the valence region of corresponding PS spectra. © 1994 John Wiley & Sons, Inc.
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  • 161
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    International Journal of Quantum Chemistry 52 (1994), S. 501-523 
    ISSN: 0020-7608
    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 mathematically based approach is made to categorize the different stereochemical possibilities for finite carbon clusters comprised of fully bonded smooth nets of carbon atoms all of which are trigonally hybridized. An exceptional richness of ideas and theorems from topology, differential geometry, and graph theory are found to be relevant and to govern not only the conventional chemical graph-theoretic molecular interconnection patterns but also topo-geometric features of the stereochemically conceivable structures and their associated strain. The usually considered fullerenes (with or without isolated pentagonal rings) form but a fraction of the overall possibilities, some of which e.g., do not require pentagonal rings (or other “small” rings). But too there emerges from the theory certain special structures which turn out often to correspond to experimentally observed fullerenes. © 1994 John Wiley & Sons, Inc.
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  • 162
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    International Journal of Quantum Chemistry 52 (1994), S. 481-500 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report some of our recent results from theoretical modeling of the interaction between metals and π-conjugated molecules. We apply the semiempirical Austin Model 1 method for the investigation of two fundamentally different systems: sodium interacting with diphenylpolyenes and aluminum interacting with poly (p-phenylenevinylene) and derivatives. In the former case, electronic-structure calculations are also performed using the nonempirical pseudopotential Valence Effective Hamiltonian (VEH) technique. For sodium interacting with diphenylpolyenes, we investigate the geometric and electronic structure modifications that are induced upon charge transfer in a series of diphenylpolyenes with an even number of carbons (from stilbene to α,ω-diphenyltetradecaheptaene, i.e., one to seven double bonds in the polyene part of the molecule). Densities of valence states generated from the VEH calculations are directly compared to experimental ultraviolet photoelectron spectroscopy valence band spectra; these are recorded during successive sodium exposure of the molecular solids. The charge-storage states in the series are discussed in terms of soliton-antisoliton-pairs and polaron-like states induced upon doping (reduction). Introducing aluminum atoms onto poly (p-phenylenevinylene) systems allows us to study the initial stages of interface formation. We find that aluminum atoms preferentially react with the vinylene linkages in both poly (p-phenylenevinylene) and poly (2,5-dimethoxy-p-phenylenevinylene). When carbonyl groups appear on the side of the chains, as in poly (2,5-dialdehyde-p-phenylenevinylene), new reactive sites are induced, leading to structures with stabilities comparable to those in the most stable configurations involving a single vinylene group. In all three systems investigated, the interaction with aluminum induces major modifications of the polymer chains with interruptions of the π-system caused by formation of sp3-like defects. © 1994 John Wiley & Sons, Inc.
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  • 163
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    International Journal of Quantum Chemistry 52 (1994), S. 541-551 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The minimum-energy structures and bonding properties of the hydrogenated lithium clusters, Li4H2 and Li7H, have been investigated by means of an ab initio Monte Carlo simulated annealing method. The minimum-energy structures of Li4H2 and Li7H are found to resemble those of the triangular planar (D3h) isomer of Li6 and Td isomer of Li8 clusters, respectively. © 1994 John Wiley & Sons, Inc.
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  • 164
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    International Journal of Quantum Chemistry 52 (1994), S. 553-563 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The energy surfaces of Mn+/SCN- and Mn+/OCN- (M = Li+ and Mg2+) ion pairs have been calculated at the Hartree-Fock and MP2 levels of theory. The electrostatic potential and the actual binding energies are compared. Besides linear ion pairs, nonlinear ones are also found and are in some cases the most stable ones. The electrostatic potential and the actual binding to cations are compared. © 1994 John Wiley & Sons, Inc.
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  • 165
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    International Journal of Quantum Chemistry 52 (1994), S. 571-577 
    ISSN: 0020-7608
    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 calculations are performed on Ga3As3, Ga3As4, and Ga4As3 clusters. Geometries, electron affinities, ionization potentials, and bondings are analyzed for these clusters. Geometry optimizations using analytic energy gradients followed by single point MP2 calculations are performed to decide for the equilibrium structure. The alternation pattern observed for the ionization potential and electron affinity are verified for these clusters after correlation energy is included. Our results are compared with previous theoretical and experimental calculations. © 1994 John Wiley & Sons, Inc.
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  • 166
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    International Journal of Quantum Chemistry 49 (1994), S. 299-308 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The physics of a system is determined by a variation of the action integral, i.e., by a variation of the space-time volume integral of the Lagrange function. If one demands that the properties of an atom in a molecule be derived from physics, the atom must generate its own space-time volume, requiring that its boundaries be defined in real space. The variations in the action are related to the actions of generators of infinitesimal unitary transformations. In the general case, the action integral is altered by generators acting in both the spacelike and timelike surface bounding the space-time volume, whereas for a total isolated system, the physics is totally determined by their action in just the spacelike surfaces at the two time endpoints. It is shown and illustrated for a one-dimensional system that the definition of an atom corresponds to the possibility of choosing a subsystem in such a way that the contributions to the change in action resulting from the evolution in time of its spatial boundaries vanishes identically. The properties of these subsystems and of the total system of which they are a part are, therefore, determined by one and the same action principle. This choice of subsystem corresponds to the possibility of augmenting the Lagrange function by the divergence of the gradient of the electron density a step that, while leaving the equations of motion unchanged, modifies the generating operators in the required manner. © 1994 John Wiley & Sons, Inc.
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  • 167
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    International Journal of Quantum Chemistry 49 (1994), S. 321-342 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: With the density functional theory as the underlying framework, the modeling of the ground-state density p(r), using fragments or bonds as appropriate, is considered in both free molecules and in some condensed phases. Emphasis is placed on bringing the models into contact with both first-principles quantum mechanics and available diffraction experiments. Clusters of alkali metals are specifically referred to, since cluster science affords a bridge between the quantum chemistry of small molecules and condensed phases. Finally, amorphous silicon and both solid and liquid alkali metals are considered. © 1994 John Wiley & Sons, Inc.
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  • 168
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    International Journal of Quantum Chemistry 49 (1994), S. 549-557 
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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 this work, a way to approximate the correlation energy functional starting from a model correlation factor is shown. The problem is addressed by using formally exact properties of the second-order density matrix and actual values of correlation energies for atoms. An Ansatz for the correlation factor is proposed that allows one to derive some known and some new correlation energy density functionals. Results for atomic systems show the reliability of the approach. © 1994 John Wiley & Sons, Inc.
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  • 169
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    International Journal of Quantum Chemistry 49 (1994), S. 591-600 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A dimensionally scaled generalization of constrained search density functional theory allows access to some simple model problems. These might be valuable for testing and perhaps improving conventional density functionals. One specific model problem is solved: When extrapolated to infinite numbers of spatial dimensions, the energy density functional for spherically symmetric, two-electron systems can be calculated to arbitrary accuracy. © 1994 John Wiley & Sons, Inc.
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  • 170
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    International Journal of Quantum Chemistry 52 (1994), S. 529-539 
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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 photodissociation process of argon cluster ion is studied by using the molecular dynamics method with non-adiabatic transitions. The potential energy surfaces and the electronic states are calculated with the diatomics-in-molecules (DIM) hamiltonian. The initial configurations are sampled from the classical trajectory paths on the potential energy surface of the electronic ground state. The non-adiabatic process is treated with the scheme proposed by J. C. Tully [J. Chem. Phys. 93, 1061 (1990)] with a slight modification. The method is applied to the photodissociation process of Ar7+. The calculated kinetic energy distribution and angular distribution of the photofragment are in good agreement with the ex-perimental results, but the branching ratio between Ar+ and Ar2+ fragment ions is not. © 1994 John Wiley & Sons, Inc.
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  • 171
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    International Journal of Quantum Chemistry 52 (1994), S. 565-569 
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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: SC-MEH-MO calculations with electron repulsion correlation and configuration interaction included, have been carried out on Sm(Cp*)2 in both its linear and bent geometrical forms. Except for relatively small factors, there are virtually no major differences in the electronic structures of the two comfomers. In both cases, the bonding is found is to be very clost to being nearly totally ionic. An estimation of the ionic bond energy is made and discussed. © 1994 John Wiley & Sons, Inc.
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  • 172
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    International Journal of Quantum Chemistry 52 (1994), S. 611-624 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A first-principles quantum molecular dynamics scheme based on density-functional theory is used to investigate the finite-temperature properties of metallic hydrogen at density 2.7 g cm-3 (rs = 1), in particular to determine melting temperature, velocity autocorrelation functions below and above melting, and the temperature dependence of proton self-diffusion in the liquid phase. A strong deviation from the prediction of the one-component plasma model is found in the value of the melting temperature. Results of simulations of the molecular phase are also presented and found to be in agreement with previous work on dense molecular hydrogen. The scheme is based, for the modeling of the electrons, on time-dependent Schrödinger equations, coupled to classical equations of motion for the protons. All numerical results were obtained using a parallel computer, and an outline of the computer program implementation is presented. © 1994 John Wiley & Sons, Inc.
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  • 173
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    International Journal of Quantum Chemistry 52 (1994), S. 195-207 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chemical and biological damage, caused by directly or indirectly ionizing radiations, is attributable to the action of the charged particle tracks in the absorbing medium. Attempts to elucidate the biophysical mechanisms involved, and to quantify the damage, are typically made in terms of one or more of the main physical parameters descriptive of the charged particle tracks. To meet a need for a ready reference source of such information, tables of the relevant parameters have been calculated for a liquid water medium. The full tables are obtainable elsewhere. Here, a description is given of the quantities calculated and an extended example is given of their application in elucidating the physical mechanisms of radiation-induced biological damage. A representative selection of data is displayed graphically to illustrate the extent of the information obtained and its value in, e.g., application to fundamental radiation dosimetry. Track structure data is tabulated for instantaneous energies of individual particles and for the fluence and dose-weighted spectra at charged particle equilibrium. Data are listed for incident electrons (50 eV to 30 MeV); characteristic Kα X-rays from carbon to uranium; commonly used radioisotope sources of 241Am, 137Cs, and 60Co and for continuous X-ray spectra (≤300 kV); Auger electron and beta-emitter radionuclides; heavy charged particles having specific energies of 0.5 keV/μ to 1 GeV/μ for 74 ion types ranging from protons to uranium ions, and for monoenergetic neutrons (0.5 keV to 100 MeV). Quantities listed are kerma factors; fluence of charged particles per unit source concentration; buildup factors; track and dose-average LET and restricted LET; W values; z2/β2;β2; delta-ray yields, energies, and ranges; ion ranges; and the mean free path for primary ionization and the linear primary ionization. For indirectly ionizing radiations, the microdose quantities, frequency, and dose means of lineal energy are tabulated along with typical energy deposition distribution spectra for neutrons and gamma rays in micron and nanometer volumes. © 1994 John Wiley & Sons, Inc.
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  • 174
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    International Journal of Quantum Chemistry 49 (1994), S. 21-24 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ketene cycloadditions give predominantly the more hindered product. This has been adduced as evidence for concerted antarafacial cycloaddition in accord with the Woodward-Hoffmann rules. However, stepwise mechanisms, heretofore discounted, are now shown to account for the products in a simple least-motion manner that requires only that rotation in the intermediates be not greatly faster than cyclization. This requirement is amply supported by the literature. © 1994 John Wiley & Sons, Inc.
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  • 175
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    International Journal of Quantum Chemistry 49 (1994), S. 45-57 
    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: It is advocated to carry out an optimization procedure, which is based upon the variational method, in such a way that the optimum values of the variational parameters are expressed as functions of physical constants, such as the atomic number, Z. The three stages involved in this treatment are illustrated by the optimization of nine correlated wave functions, which describe the ground states of atomic two-electron systems. An analysis of the Z-expansions of the total energies associated with these functions leads to the concept of a class of variational functions. The performances of functions belonging to the same class differ only marginally, especially at larger values of Z. Consequently, the concept of class may be used to bring some order in the plethora of variational functions. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 49 (1994), S. 575-579 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An analysis is presented of the results of earlier ab initio computational studies of cyclobutadiene, cyclooctatetraene, and 1,4-dihydropyrazine. The first and third of these are normally categorized as antiaromatic. All three molecules are polyenes, even when the last two are forced into planar conformations. There is no driving force for extensive π delocalization, even when it would appear to have been facilitated. Calculated isodesmic energies show a net destabilization only in the case of cyclobutadiene, which we attribute to strain and repulsion between the π electrons of the C=C double bonds. The other two molecules have negative isodesmic energies, indicative of net stabilizing effects. We conclude that the concept of antiaromaticity is useful for identifying molecules that resist the apparent opportunity for extensive © delocalization, but that it does not intrinsically imply net destabilization. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 49 (1994), S. 601-612 
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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 present a method closely related to configuration interaction over single excitations (CI-S) for the calculation of static dipole moments and polarizabilities, which we call modified configuration interaction over single excitations (MCI-S). This method allows for orbital relaxation through the inclusion of single excitations on a correlated first-order Rayleigh-Schrödinger wave function. An approximation to our MCI-S results in an inexpensive method to calculate polarizabilities that yields good results when compared with the full configuration interaction (CI-F) results. This method, called modified configuration interaction over single excitations without triple excitations (MCI-S_WT), seems particularly suitable to applications for large systems. Examples are presented using the intermediate neglect of the differential overlap model Hamiltonian (INDO/1), which indicates that the MCI-S_WT procedure presents an error of less than 15% with respect to the CI-F values in a fraction of the computational effort. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 49 (1994), S. 639-648 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chemical signed graph theory is presented. Each topological orbital of an N-vertex molecular graph is represented by a vertex-signed graph (VSG) that is generated by assigning a sign, either plus or minus, to the vertices without solving the secular matrix equation. The bonding capacity of each VSG is represented by its corresponding edge-signed graph (ESG) and is quantified by the net sign of the ESG. The resulting 2N-1 configurations of VSGs can be divided into several groups according to the net signs of the corresponding ESGs. Summation of an appropriate set of degenerate VSGs is found to yield the conventional, canonical molecular orbitals. The distribution of the number of VSGs with respect to the net sign is found to be binomial, which can be related to bond percolation in statistical physics. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 49 (1994), S. 705-725 
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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 generalized nonlinear Schrödinger equation has been solved in order to understand how electronegativity and the covalent radius of an atom vary during a chemical reaction. The chemical reaction is modeled as an atom in an external field and also as an atom colliding with an ion. Temporal evolution of charge density, current density, and effective potential has also been studied for a better understanding of the process. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 49 (1994), S. 739-770 
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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
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  • 181
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    International Journal of Quantum Chemistry 50 (1994), S. 43-53 
    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: The pairon population analysis based on the geminal expansion of pair densities is introduced and applied. As demonstrated by numerical data calculated for a series of simple molecules by the semiempirical MNDO method, the resulting populations provide a new simple means of visualizing the molecular structure. In addition to the reproduction of classical structural formula including the multiplicity of individual bonds, the resulting populations confirm the transferability of bond energies and also provide a simple interpretation of the concept of quantum chemical valence. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 50 (1994), S. 55-67 
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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, improved algorithms for evaluating the irreducible representations of the symmetric group associated with an arbitrary partition, such as the orthogonal, the natural, and the seminormal representation, are introduced and the relations among them are discussed. With the new algorithms, a representation matrix for the orthogonal or the seminormal representation is expressed as the product of three matrices, where two of them are the triangular intrinsic matrices depending on the irreducible representation of the symmetric group; the other relating to the permutation can be given explicitly. Furthermore, we give a concise description for the irreducible representations of the symmetric group and reach an interesting conclusion that the conjugation transformation matrix between the orthogonal and the natural representation is the intrinsic matrix of the symmetric group. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 49 (1994), S. 147-157 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electron-pair (intracule and extracule) densities of the first-row hydrides may be understood on the basis of the different chemical-bonding schemes in this series. Thus, the LiH pair densities show a strongly ionic nature and may be fairly well described by Li+H-. The CH-pair densities, on the other hand, may be approximated by the promolecular superposition of a carbon and a hydrogen atom with an accumulation of pair density in the internuclear region signifying covalency. In the case of FH, its pair densities show a predominately ionic structure and are closer to those of F- than to those of the promolecular superposition of a fluorine and a hydrogen atom. The slight deformation of longitudinal pair densities observed in FH is largely due to the presence of the H+. © 1994 John Wiley & Sons, Inc.
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  • 184
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    International Journal of Quantum Chemistry 50 (1994), S. 189-196 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The valence bond method has been implemented for the study of π-electron systems using a semiempirical CNDO-type Hamiltonian. The algorithm used is based on the Clifford algebra realization of the Rumer-Weyl basis presented by Paldus et al. Using this version of the CNDO-VB approach, the effect of the metal hybridization in the electronic delocalization of metallacyclobutadienes is discussed. © 1994 John Wiley & Sons, Inc.
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  • 185
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    International Journal of Quantum Chemistry 49 (1994), S. 215-237 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We introduce generalized bond orders defined in terms of weighted Kekule valence structures. The weights were determined by the contributions of linearly independent and minimal conjugated circuits in individual Kekule valence structure. When special values for the contributions of conjugated circuits of different size are assumed, one obtains quantities that show considerable similarity to the Pauling and the Clar's bond orders. Pauling bond orders are obtained when one assumes that all conjugated circuits make equal contribution to bond orders. © 1994 John Wiley & Sons, Inc.
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  • 186
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    International Journal of Quantum Chemistry 49 (1994), S. 239-251 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chemical binding is modeled through interatomic charge transfer and accumulation of electron density at the bond center using the concepts of electronegativity and hardness parameters defined for the bond region. The generalized electronegativity and hardness parameters for the up- and down-spin electrons are also defined within the framework of spin-polarized density functional theory, leading to the formulation of covalent binding in molecules in terms of a two-way flow of unpaired electrons between the atoms. The associated energy changes corresponding to these descriptions are shown to provide quite accurate predictions of bond energies for simple heteronuclear diatomic molecules. © 1994 John Wiley & Sons, Inc.
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  • 187
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    International Journal of Quantum Chemistry 49 (1994), S. 291-298 
    ISSN: 0020-7608
    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 time, we obtain practical density matrices approximately N-representable by correlated-determinant wave functions, which are functionals of the electron density and entirely defined by information obtainable from the X-ray coherent diffraction experiment. © 1994 John Wiley & Sons, Inc.
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  • 188
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    International Journal of Quantum Chemistry 49 (1994) 
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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
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  • 189
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    International Journal of Quantum Chemistry 49 (1994), S. 353-361 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The spin virial theorem is derived in the density functional theory. The theorem establishes a relation between the differences of spin-up and -down kinetic and potential energies. The theorem is useful for checking the accuracy of spin orbitals. As an illustration, the example of the Xα method is studied. © 1994 John Wiley & Sons, Inc.
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  • 190
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    International Journal of Quantum Chemistry 49 (1994), S. 397-407 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electrostatic charge models for molecules have been developed by employing the critical topographical features of the molecular electrostatic potential (MESP) as the “fitting” criterion. These models include one or more spherical Gaussians for incorporating the continuous electron-charge distribution in addition to the positive valued point charges representing the nuclei. The model parameters (point charges, the orbital exponents, and Gaussian centers) are optimized so as to mimic the extremal characteristics of the corresponding quantum chemical MESP. The test cases reported here include methane, ethylene, and methanol molecules. The charge models developed using the present method are seen to satisfactorily reproduce the ab initio MESP and its extremal features. © 1994 John Wiley & Sons, Inc.
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  • 191
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    International Journal of Quantum Chemistry 49 (1994), S. 383-396 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structure and properties of Wheland's intermediates are studied in some model systems. The effect of the annelated small ring is simulated by bending two vicinal CH bonds in benzene toward each other. The extent of bending describes the ring-size effect of a small fused fragment molecule. The electrophilic substituent is represented by a proton. Calculations at the HF/6-31G* and MP2/HF/6-31G* levels of sophistication confirm the Mills-Nixon (MN) hypothesis. It is conclusively shown that ß-complexes are more stable than are α-protonated species. The results are interpreted in terms of the ground-state density distribution dictated by rehybridization and of the transition structure π-electron redistribution triggered by protonation. Their effects are additive to a large extent as far as CC bond distances are concerned. Energetic properties are determined by the interplay of two characteristic π-electron localization patterns, one caused by the initial angular strain and the accompanying rehybridization related to the ground state and the other one occurring during formation of the proton σ-complex. This interplay destabilizes α-intermediates relative to ß-intermediates. © 1994 John Wiley & Sons, Inc.
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  • 192
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    International Journal of Quantum Chemistry 49 (1994), S. 449-461 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quadrupole and octupole Cauchy moments of the atoms through argon are calculated using the hydrodynamic formulation of time-dependent Kohn-Sham theory. The exchange-correlation energy density functional is approximated by a gradient expansion for atoms that has an explicit dependence upon the number of electrons. The first-order corrections to the Kohn-Sham amplitudes and phases are found by seeking variational solutions of the derived sequential set of functionals. The trial functions employed contain both linear and nonlinear variational parameters and are thus flexible enough to provide rapid convergence to the multipole polarizabilities. The resulting Cauchy moments provide information that allows the calculation of various properties that result from the linear interaction of atoms with a time-varying electric field. © 1994 John Wiley & Sons, Inc.
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  • 193
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    International Journal of Quantum Chemistry 51 (1994), S. 79-85 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Through a new local density approximation to the kinetic energy density functional introduced by us recently, a simple Thomas-Fermi-like scheme for the direct calculation of electron density in atoms is proposed. The calculated density is nonsingular at the nucleus and the energy values are in very good agreement with the corresponding Hartree-Fock results for atoms. © 1994 John Wiley & Sons, Inc.
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  • 194
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    International Journal of Quantum Chemistry 51 (1994), S. 105-112 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The influence of accepting the hydrogen bond by N(3) of adenine on the conformational preferences of the N(6) substituent in modified nucleic acid base N6-(N-glycylcarbonyl)adenine (gc6Ade) has been modeled by the protonation of N(3). The preferred orientation of the glycylcarbonyl substituent changes on the protonation of N(3). The preferred conformation for the N(3) protonated base is the planar, intramolecular bifurcated hydrogen-bonded structure involving the interaction of the ureido N(11)H with the N(1) of the purine and the O(13b) of the amino acid carboxyl. Another conformation of nearly equal stability, having the internal hydrogen bonding of O(13b) with the N(6)H, is also predicted. Such protonation or hydrogen-bonding-induced conformational flipping may enable the structural reorientation of the anticodon loop required for the functioning of tRNA. © 1994 John Wiley & Sons, Inc.
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  • 195
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    International Journal of Quantum Chemistry 51 (1994), S. 123-133 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The important duality problem of phase-space representations of quantum mechanics is discussed and the class of self-dual representations is established. It is shown that self-dual phase-space representations may be endowed with a Hilbert space structure resembling the standard picture of quantum mechanics. A stationary variational principle is then formulated, enabling one to solve eigenproblems of observables directly in phase space. A modified, more usable form of this variational principle is also given together with an example of practical application to the 1D Coulomb system. © 1994 John Wiley & Sons, Inc.
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  • 196
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    International Journal of Quantum Chemistry 51 (1994), S. 141-159 
    ISSN: 0020-7608
    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 dimer form of 1,3-dimethyluracil was carried out. The optimum geometric parameters in the monomeric and dimeric forms were obtained using the AM1 semiempirical method. The results were compared with those reported by X-ray diffraction. The energies in the dimer formation and in the intramolecular C—H···O interaction were calculated. Electronic density maps in two and three dimensions were drawn. Several calculated thermodynamic parameters are discussed. © 1994 John Wiley & Sons, Inc.
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  • 197
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    International Journal of Quantum Chemistry 49 (1994), S. 835-848 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A derivation of the linear response function for the Brueckner coupled cluster method is presented that enables the calculation of second-order molecular properties such as frequency-dependent polarizabilities. By using the Brueckner orbital variant of coupled cluster theory, the spurious pole structure inherent in the standard coupled cluster approach with orbital relaxation is avoided. © 1994 John Wiley & Sons, Inc.
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  • 198
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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
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  • 199
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    International Journal of Quantum Chemistry 49 (1994), S. 849-875 
    ISSN: 0020-7608
    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 d(AG) · d(CT), d(TG) · d(CA), d(AC) · d(GT), and d(TC) · d(GA), which are stacked base pairs in the DNA double helix, are elucidated for A and B conformations in detail by DV-Xα cluster calculations. By using the S̃r values of the O2 (and O3) of each phosphate in the clusters together with those calculated in previous articles, it becomes possible to treat chemical reactivities of phosphates as recognition sites for all DNA fragments. Contour maps of the wave function of the HOMO and LUMO for bases differ subtly among the clusters and show the various fields around the P ion induced by the respective stacking constructed from four kinds of bases. © 1994 John Wiley & Sons, Inc.
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  • 200
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    International Journal of Quantum Chemistry 50 (1994) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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