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  • 1990-1994  (1,618)
  • Atomic, Molecular and Optical Physics  (1,617)
  • Nuclear reactions
  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 727-738 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Hubbard model is rederived from a tight-binding band calculation, showing that, as long as parameters are properly chosen, the model is justified for calculating electronic properties of narrow-band systems. The treatment is extended by including correlations and it was found that bound solutions called dimers exist. The concept of dimers is found to be very powerful for understanding the unusual properties of heavy fermion systems. However, a Mott-Hubbard-like model may be required to calculate properties of high Tc cuprates. © 1994 John Wiley & Sons, Inc.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 771-771 
    ISSN: 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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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (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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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 781-788 
    ISSN: 0020-7608
    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 electronic structures calculations are reported for the four low-lying electronic states X 2B1, 2B2, 2A2, and 2A1 of the CH2NO2 radical. The geometric parameters for the ground-state X 2B1 are predicted by MRSDCI calculations with a double zeta plus polarization basis set. The vertical excitations energies for these electronic states are determined using MRSDCI/DZ+P calculations at the ground-state equilibrium geometry and in agreement with the recent experimental data obtained via PES of the CH2NO2- anion. The oscillator strenghts and the radiative lifetimes for these electronic states and the spin properties for the ground state are calculated based on the MRSDCI wave functions, predicting results in good agreement with available experimental data. © 1994 John Wiley & Sons, Inc.
    Additional Material: 4 Tab.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 789-804 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A many-fermion interacting system is investigated within the scenario of the Feynman path integral representation of quantum mechanics. Short-time propagator algorithms and a basis set, closely related to the coherent states, are used to obtain the many-body analytic propagator. A second-quantized Hamiltonian involving a restricted set of two-body interactions and the whole set of Coulomb interactions are separately and shown to lead to an exact and an approximate propagator, respectively. In the latter case, use of a grand canonical ensemble allows the grand partition function and the density operator matrix to be readily obtained. No further approximations are required in the calculation of the trace of the evolution operator involved in the evaluation of statistical expectation values. © 1994 John Wiley & Sons, Inc.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 773-780 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The problem of the determination of eigenvalues for two coupled Schrödinger equations is considered. A new method to solve this problem is presented. This method replaces the use of the wave functions (with unknown initial values) by eight canonical functions αij and βij (i = 1,2; j = 1,2) having well-defined initial values at an arbitrary “origin” r0. These functions are collected in four couples; each one is the solution of the given coupled equations. For a given E, an “eigenvalue functions” D(E) is defined by an analytical expression depending on αij (r) and βij (r) at r = 0 and r = ∞ only. The successive eigenvalues En of the given system are precisely the successive intersection of the graph D(E) with the E-axis. The present method eliminates the conventional use of wave function initial values as well as the conventional problem of the prior guess of the limit points; it determines these points automatically. It eliminates also the use of trial values for E and the need of iterations for its correction. The numerical application of a standard example used by Friedman and co-workers (1990) shows that the eigenvlues computed by the present method are highly accurate for low and high levels; the average relative discrepancy between computed and exact levels is about 3.4 × 10 -15 (this discrepancy never exceeds 1.6 × 10 -14), which is almost the precision of the computer. © 1994 John Wiley & Sons, Inc.
    Additional Material: 1 Tab.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 805-816 
    ISSN: 0020-7608
    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 of some benzylidene, arylethylidene, and heterocyclic arylidene malononitriles has been calculated within the framework of the MNDO-MO formalism. Various structural factors are analyzed and discussed in terms of localized and delocalized MOS and π-interaction between methylene malononitrile (MMN) and aryl moieties. The possibility of charge transfer (CT) from aryl to MMN moieties has been examined. The presence of a heterocyclic ring introduces a small perturbation into the MMN nuclear frame. p-Substituents have a pronounced effect on the magnitude and direction of the dipole moment. The variation of ionization potentials and bond order with the Hammet σ p are examined and a straight-line relationship is obtained. The correlation between physiological activity and quantum mechanical properties is analyzed in terms of heat of formation, ionization potential, dipole moment, and charge density on the aryl ring. © 1994 John Wiley & Sons, Inc.
    Additional Material: 7 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 817-833 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Contracted split valence 311 Gaussian basis sets optimized for the averaged relativistic effective potentials (AREP) of Christiansen et al. are developed for the main-group elements B—Ne, Al—Ar, and Ga—Kr. These triple-zeta basis sets are intended for use in valence-only molecular calculations with inclusion of correlation. The performance of the optimized basis functions is first analyzed in HF atomic calculations and compared with HF-limit results. Molecular calculations at the HF, MP2, and CISD levels of theory are performed for some selected diatomics, using these potentials and the optimized sets supplemented by one standard d-type polarization function. Results are reported for bond lenghts, harmonic vibrational frequencies, dipole moments, atomization energies, and ionization energies and compared with all-electron reference calculations and experimental data. © 1994 John Wiley & Sons, Inc.
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  • 10
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 879-881 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 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
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  • 13
    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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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 1-20 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rules for molecular cyclicity based on the global indices resulting from reciprocal distances (Harary number, H) or from resistance distances (Kirchhoff number, Kf) were tested in comparison with those elaborated earlier by means of the Wiener index, W. The Harary number and the Wiener number were found to match molecular cyclicity in an almost identical manner. The Kirchhoff number also generally follows cyclicity trends described previously. H is slightly less degenerate than is W, but Kf has practically no degeneracy in the graphs investigated here. Being much more discriminating than the Wiener number (i.e., practically nondegenerate), Kf allowed the formulation of new rules for systems formed from linearly condensed ribbons of even-membered rings with different sizes as well as for branched ribbons. The topological cyclicity patterns are thus reformulated in an extended basis, proceeding from three different graph metrics. © 1994 John Wiley & Sons, Inc.
    Additional Material: 8 Tab.
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 55-67 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 69-90 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations at 6-31G**, 6-31++G**, and MP2/6-31G** levels were performed on disilyl-fluoronium, (SiH3)2F+, with the SiH3 group eclipsed or staggered. Optimized geometries, total energies, dipole moments, atomic charges, electronic density, and vibrational frequencies were computed. The results were compared with calculated structural parameters and vibrational frequencies of H3SiF, H2SiF+, H2SiF-, and H4SiF+ ions. The basis-set effects were studied. Several thermochemistry parameters - ZPE, thermal energy, rotational constants, and entropies - were also calculated. © 1994 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
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  • 18
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 91-91 
    ISSN: 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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  • 19
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 93-99 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The mechanism of the cycloaddition reaction between ketene and allene to form methylene-cyclobutanones has been studied theoretically by HF/3-21G and MP2/3-21G. These two reactions are believed to be unsynchronous and concerted, taking place through the twisted transition states. Four orbitals are mainly involved in each reaction, which is a “2 × [1 + 1]”-type cycloaddition. The activated barrier for the two reactions are 27.2 and 27.1 kcal/mol, respectively, at the level of MP2/6-31G* based on the MP2/3-21G geometries, i.e., these two reactions are compatible. © 1994 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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  • 21
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 101-107 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The derivation of the connected moments expansion (CMX) is examined as well as the singularities that arise in the series expansion for the ground-state energy. Explicit analytic results are presented that show a canceling of these singularities. Also, an alternate moments expansion (AMX) is derived that closely models the CMX but displays a varied computational range. © 1994 John Wiley & Sons, Inc.
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  • 22
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have extended our computations of the structure and of the infrared and Raman spectra of methylphosphonates and related compounds to the O-ethyl S-2-diisopropylaminoethylmethylphosphonothiolate molecule (we abbreviate the name to ESD). We have computed the optimized geometry and the vibrational infrared and Raman frequencies of ESD by means of the Guassian 92 Program Package using 6-31G* basis sets. We assign the vibrational frequencies and we correct each frequency by multiplying it with a previously derived 6-31G* correction factor. The result is a computer-generated prediction of the IR and Raman spectra of ESD. The agreement between our theoretical predictions and the experimental IR spectrum of ESD is surprisingly good. © 1994 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 23
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 173-179 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure of B14, B142-, and B14H142- in octahedral symmetry has been investigated by ab initio calculations at the STO-3G and 4-31G levels. The relationship of molecular orbitals among them has been analyzed and it can be found that the number of valence bonding orbitals of high borane obeys the Wade rule. The similarities and difference between boron clusters and carbon clusters are also discussed. © 1994 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 24
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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.
    Additional Material: 4 Ill.
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  • 25
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 197-205 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Based on the virial theorem and the virial function, a description of the orbital binding effect and the definition of orbital bond order are developed within the framework of molecular orbital theory for both diatomic and polyatomic molecules. Orbital binding effect calculations for N2, CH4, C2H4, C2H2, and C4H4 molecules show that the definition is reasonable. © 1994 John Wiley & Sons, Inc.
    Additional Material: 5 Tab.
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  • 26
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 181-188 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Catalysts based on oxides of transition metals were studied by Xα-DV calculations. The chemical composition and electronic structure of surface layers for platinum(IV) oxide catalysts modified under percompound electrosynthesis were determined by X-ray photoelectron spectroscopic, quantum chemical, and electrochemical data. The main regularities in electronic structure change under the transition from solid pure oxide PtO2 to its, in part, N-substituted PtO2-xNx were analyzed. Then, we looked for perspective catalysts, calculating the electronic structure for analogous compounds of Ir(III), Ir(IV), Rh(III), and Pd(II). We found that the changes in electronic structure of rhodium oxide under O - N-substitution allowed us to predict the excellent properties of its compound as a catalyst for percompound electrosynthesis reactions. © 1994 John Wiley & Sons, Inc.
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  • 27
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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
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  • 28
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 207-231 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this article, we demonstrate a complementarity between the quasi-spin SU(2) algebra of the Hubbard model and the pseudo-orthogonal group O(m,m), where n = 2m is the number of lattice sites. It is shown that all N-electron states for the one-dimensional Hubbard model, corresponding to given values of spin and quasi-spin, give rise to an irreducible representation of O(m,m). Moreover, the cyclic group Cn symmetry of the Hamiltonian is investigated and the O(m,m) ↓ Cn branching rules are determined with the use of the U(n) q-dimension formula. © 1994 John Wiley & Sons, Inc.
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  • 29
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 233-242 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Based on the EHMO approach, an approximate treatment of electronic energy-band structures is suggested. By employing this treatment, computations of the band structures for the Al-doped superconductors YBa2Cu3-xAlxO7 + δ were carried out. It is shown by analysis of the band structures and the density of states that the 2D Cu-O planes in the Y—Ba-Cu—O superconducting system play a dominant role in superconductivity, whereas the 1D Cu—O ribbons have indirectly an influence on superconductivity through the connection of the O(4) atoms to two Cu—O planes. © 1994 John Wiley & Sons, Inc.
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  • 30
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    International Journal of Quantum Chemistry 50 (1994), S. 243-271 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The superdirect configuration interaction (Sup-CI) method has the usual versatility and stability of the CI methods with computational efficiency typical to that of the many-body methods, such as the many-body perturbation theory (MBPT). The Hamilton operator is projected into a space of a few trial vectors, such as Krylov, Nesbet, or Møller-Plesset correction vectors. In this space, Hamiltonian matrix elements may be directly computed in the many-body fashion, as weighted sums of integral products over orbital indices. The variation-perturbation method based on the first-order wave function is equivalent to the Sup-CI method with a single correction vector of the Møller-Plesset type. Different points of view on the superdirect CI method are discussed and a version in which third-order contributions are computed for a relatively small (10-100) space of reference and correction vectors is tested. Selection of the best “effective first-order spaces” and size-extensivity corrections in Sup-CI are briefly discussed. Møoller-Plesset, Epstein-Nesbet, and other correction vectors are included in the model calculations on the symmetric stretch of bonds in water, acetylene, and the NH2 molecule. Errors are almost independent of molecular geometry and the method appears to be superior than the multireference second-order perturbation methods. © 1994 John Wiley & Sons, Inc.
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  • 31
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 273-277 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An isodesmic energy analysis has been carried out at the MP2/6-31G*//HF/3-21G level for the nonplanar ground state (1) of 1,3,5,7-cyclooctateraene and for two planar forms, one having complete π delocalization (2) and the other having alternating single and double bonds (3). 1 is found to have a considerable degree of stabilization, which is attributed to limited π delocalization. The polyene 3 is the more stable of the two planar forms; it is a transition state in the inversion between two possible nonplanar structures. 2 is found to be a triplet at the Hartree-Fock level and is a critical point on an alternate pathway between the two possible arrangements of alternating single and double bonds in 3. Both 2 and 3 have negative isodesmic energies, indicating the presence of stabilizing factors. Our results for 3 show that an “antiaromatic” system need not necessarily show a net destabilization. © 1994 John Wiley & Sons, Inc.
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  • 32
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    New York, NY : Wiley-Blackwell
    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
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  • 33
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    International Journal of Quantum Chemistry 50 (1994), S. 279-292 
    ISSN: 0020-7608
    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 configuration interaction (CI) calculations have been performed for the O2 + H2 complex in a trapezoidlike collision arrangement with C2v symmetry. The potential energy surfaces of the four lowest states of this van der Waals complex (arising from the X3 Σg-, a1 Δg, and b1 Σg+ states of the oxygen moiety), as well as the collision-induced b1 Σg+ - a1 Δg electric dipole transition moment (Mb-a), have been analyzed for different CI expansions, using as a reference determinant the restricted open-shell Hartree-Fock (ROHF) function for the ground state of the complex H2(X1 Σg+) + O2(X3 Σg-). The geometry optimized at the ROHF/6-311G** level was refined by a partial optimization at the CI level scanning the intermolecular distance. The equilibrium distances for the X, a, and b states have been found to be a slightly different in the region 3.02-2.98 Å. The larger binding energy of the b1 Σg+ state (2.96 kJ/mol) in comparison with the a1 Δg (2.1 kJ/mol) and ground X3 Σg- states (1.35 kJ/mol) presumably could be explained as resulting from charge-transfer interactions. A good convergence of the calculated transition moment Mb-a for the larger CI expansions (approximately 50,000 configuration-state functions) has been obtained. The calculated collision-induced intensity of the b1 Σg+-a1 Δg and a1 Δg-X3 Σg- transitions in molecular oxygen are in reasonable agreement with recent experimental data for several foreign gases. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 50 (1994), S. 293-315 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The coefficients of fractional parentage (CFP), treated by tradition as antisymmetrizing multipliers, do not exist as such in the second-quantization representation, because, therein, by definition, there is nothing to antisymmetrize. This by no way means that the above-mentioned representation is less relevant in theoretical atomic spectroscopy than is any other one. On the contrary, all the well-known mathematical apparatus is still successfully applied, alongside with numerous new useful features. Moreover, the analysis presented here yields some results not so obvious from the point of view of the coordinate representation, namely, the consequent application of the second-quantization representation led us to an expansion of many-shell wave functions employing analogs of many-shell CFP, which appeared to be considerably simpler than the traditional CFP expansion. Also, a generalization of Redmond's formula for the case of many-shell CFP and a straightforward method of deriving summation rules for products of CFP are clear in the second-quantization representation and are sketched here. © 1994 John Wiley & Sons, Inc.
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  • 35
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    International Journal of Quantum Chemistry 50 (1994), S. 317-332 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present a systematic comparison of the correlation contribution at the level of the second-order polarization propagator approximation (SOPPA) and MP2 to the static dipole polarizability of (1) Be, BeH-, BH, CH+, MgH-, AIH, SiH+, and GeH+; (2) BH3, CH4, NH3, H2O, HF, BF, and F2; and (3) N2, CO, CN-, HCN, C2H2, and HCHO. Fairly extended basis sets were used in the calculations. We find that the agreement with experimental values is improved in SOPPA and MP.2 over the results at the SCF level. The signs and magnitudes of the correlation contribution in SOPPA are similar to those obtained in analytical derivative MP2 calculations. However, it is not possible to say, in general, which method gives the largest correlation contribution or the best agreement with experiment, nor is it possible to make a priori prediction of the sign of the correlation contribution. For the first group of molecules, which have a quasi-degenerate ground state, additional CCDPPA and CCSDPPA calculations were performed and compared with polarizabilities obtained as analytical/numerical derivatives of the CCD and CCSD energies. The CCSDPPA results were found to be in better agreement with other calculations than were the SOPPA results, demonstrating the necessity of using methods based on infinite-order perturbation theory for these systems. © 1994 John Wiley & Sons, Inc.
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  • 36
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    International Journal of Quantum Chemistry 51 (1994), S. 173-180 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Within the framework of the AMO-EHF approximation for the Hubbard model, it is shown that the band gap ΔE of an arbitrary regular alternant 1D π-system (polymer), having a singlet ground state, is different from zero, i.e., the ground state is a dielectric one. At least one of the three components of the energy gap - the topological, Δtop; the geometrical, Δgeom; or the correlation, Δcorr (combined via the equation ΔE = {Δcorr2 + (Δtop + Δgeom))2}1/2 - is different from zero. Numerical results for several 1D alternant polymers with conjugated π-systems are given. © 1994 John Wiley & Sons, Inc.
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  • 37
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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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  • 38
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    International Journal of Quantum Chemistry 52 (1994), S. 177-189 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using the Hartree-Fock and MP2 methods with bases of up to 6-31++G(2d, 2p) quality, the optimum geometry of the 1:2 and 2:1 (H2O)n… (HF)m complexes of water and hydrogen fluoride is searched in a systematic way. Two minimum-energy conformations are found for the 1:2 complex connected through a low-energy transition state. For the 2:1 complex, only one minimum-energy structure is obtained. The analysis of the geometries of the minima and their vibrational frequencies shows that none of them can be used to explain the existence of the H …F—H reverse complex detected experimentally. © 1994 John Wiley & Sons, Inc.
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  • 39
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    International Journal of Quantum Chemistry 52 (1994), S. 227-245 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The dynamical behavior of simple nonrigid molecules still attracts much interest both from experimentalists and theoreticians. On the one hand, modern laser-spectroscopic techniques like SEP allow for the detection of highly excited vibrational-rotational states of a molecule, and advanced theoretical methods, on the other hand, are more and more able to calculate accurate potential energy surfaces and to simulate the intramolecular dynamics. The aim of the present article was to contribute to the understanding of the dynamical properties of simple floppy molecules by means of a comparative study of the two triatomics HCN and HO2. Continuing our earlier work, we start from an analysis of the potential energy surface topography, then we investigate the classical dynamical behavior and the localization of the quantum states. Several conclusions of probably more general validity are drawn. © 1994 John Wiley & Sons, Inc.
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  • 40
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    International Journal of Quantum Chemistry 52 (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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  • 41
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    International Journal of Quantum Chemistry 52 (1994), S. 267-280 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The elongation method, a theoretical tool to synthesize the electronic states of polymers, is applied within the framework of the density functional approach and using a linear combination of Gaussian-type orbitals. In this treatment, the wave function of a cluster is localized and the interaction with an attacking monomer is self-consistently calculated according to the Kohn-Sham equation. The reliability and the applicability of our treatment are examined by the application to a random hydrogen molecule cluster, comparing the results with those obtained by the usual diagonalization method for the whole system. The results show that this treatment efficiently provides the electronic states of the end part of aperiodic polymers. © 1994 John Wiley & Sons, Inc.
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  • 42
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    International Journal of Quantum Chemistry 52 (1994), S. 301-308 
    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: A new approach to the description of the formation of spherulites in a polymer solution is proposed. It is based on an analytical scheme that takes into account the mass conservation law as a fundamental evolution equation. Three physically interesting cases are considered, both which, on deterministic and stochastic levels, can reflect an asymptotic behavior characteristic for spherulites, namely, R(t) ∝ t, where R(t) is a radius of the spherulite measured at instant t. A few examples of systems similar to that under study are mentioned. © 1994 John Wiley & Sons, Inc.
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  • 43
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    International Journal of Quantum Chemistry 52 (1994), S. 309-319 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An elongation method, which had already been proposed to calculate the electronic structure of aperiodic polymer efficiently, was applied to atactic polypropylene, with the approximation level of the extended Hückel method. The atomic populations thus calculated were found to be dependent sensitively on the tacticity of the polymer. Next, this elongation method was applied to the π-electron stacking system composed of ethylene and butadiene, etc. Excellent agreement was found between the elongation method and the usual extended Hückel calculations. Thus, the elongation method can confidently be applied to π-electron molecular crystal systems. Finally, the local density of states of polyacetylene with both cis-transoid and trans-cisoid structures was calculated. We found that the site with large local density of state in the valence band of a polymer chain gives large electron transfer to the site with large local density of state in the conduction band of another polymer chain. © 1994 John Wiley & Sons, Inc.
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  • 44
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    International Journal of Quantum Chemistry 52 (1994), S. 339-352 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Modifications of Witten-Sander diffusion-limited aggregation (DLA) were proposed. Effects caused by releasing boundary shape, nonfixed seed, and rotating biased drift on the morphology of the resulting DLA were examined. The diffusion-limited reaction on surfaces of DLA were also studied by performing a multifractal scaling analysis. © 1994 John Wiley & Sons, Inc.
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  • 45
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    International Journal of Quantum Chemistry 52 (1994), S. 425-435 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The multiparticle correlation expansion for the entropy of a classical monatomic liquid is presented. This entropy expresses the physical picture in which there is no free particle motion, but, rather, each atom moves within a cage formed by its neighbors. The liquid expansion, including only pair correlations, gives an excellent account of the experimental entropy of most liquid metals, of liquid argon, and of the hard-sphere liquid. The pair correlation entropy is well approximated by a universal function of temperature. Higher-order correlation entropy, due to n-particle irreducible correlations for n ≥ 3, is significant in only a few liquid metals, and its occurrence suggests the presence of n-body forces. When the liquid theory is applied to the study of melting, we discover the important classification of normal and anomalous melting, according to whether there is not or is a significant change in the electronic structure upon melting, and we discover the universal disordering entropy for melting of a monatomic crystal. Interesting directions for future research are extension to include orientational correlations of molecules, theoretical calculation of the entropy of water, application to the entropy of the amorphous state, and correlational entropy of compressed argon. We clarify the relation among different entropy expansions in the recent literature. © 1994 John Wiley & Sons, Inc.
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  • 46
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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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  • 47
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    International Journal of Quantum Chemistry 49 (1994), S. 63-66 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 48
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    International Journal of Quantum Chemistry 49 (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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  • 49
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    International Journal of Quantum Chemistry 49 (1994), S. 67-80 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The screened-Coulomb exchange model used first in the Xα method and later on in Kohn-Sham density functional theory is reexamined. Based on the well-elaborated framework of the local spin-density approximation, we show that this model is not well suited when systems with a finite number of electrons are concerned, because it does not respect the pair-density sum rule. A proper modification of the model is proposed by reformulating it in terms of a screened-exchange hole and ensuring the sum rule for this hole. As a result, it is shown how the static screened exchange in finite systems is accompanied by a conjugate antiscreened one. The possible consequences of this effect on the application of the screened-Coulomb exchange model are discussed. © 1994 John Wiley & Sons, Inc.
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  • 50
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    International Journal of Quantum Chemistry 49 (1994), S. 81-86 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A formula of the coefficient aN-K of the eigenpolynomials connected with the zero-order term's coefficient of the eigenpolynomials corresponding the k-order-induced subgraph's molecular fragments has been induced. From this formula, aN-K can be calculated and the contributions of the induced subgraph's molecular fragments to the stability and reactivity of the molecules are revealed. © 1994 John Wiley & Sons, Inc.
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  • 51
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    International Journal of Quantum Chemistry 49 (1994), S. 87-95 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The concept of internal connectivity associates to each eigenvector of the molecular graph a graph with signed edges. We show that the respective edge signs can be deduced without knowing the graph eigenvectors. The dependence of the edge signs on molecular structure is elucidated. The sign of an edge whose end vertices are νr and νs is found to be the result of separate interactions through all paths connecting the vertices νr and νs. The analysis of the edge signs is particularly simple in the case of acyclic systems. © 1994 John Wiley & Sons, Inc.
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  • 52
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    International Journal of Quantum Chemistry 49 (1994), S. 97-104 
    ISSN: 0020-7608
    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 levels of small titanium oxide clusters [(TiO2)2, and (TiO2H)3, and (TiO2H)2] have been calculated using ab initio SCF methods. Both crystal and relaxed geometries have been considered. Systematic changes in the valencelevel structure resulting from geometry relaxation are found, which may be related to band-gap changes experimentally observed for small titanium oxide particles. In addition, a Ti—OH local surface state is found to be well described within a limited cluster model. © 1994 John Wiley & Sons, Inc.
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  • 53
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    International Journal of Quantum Chemistry 49 (1994), S. 145-146 
    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
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  • 54
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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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  • 55
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    International Journal of Quantum Chemistry 49 (1994), S. 185-195 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: As the representative point on an electronic energy surface moves along any closed level curve, the corresponding electronic eigenvectors behave as does the normal to a Moebius strip when its foot is moved around the strip. This introduces a topology into the space of eigenvectors that is not envisaged in the definition of Hilbert space, but is fundamental to molecular quantum mechanics. Salient properties of these “non-Hilbertian” vector spaces are established. © 1994 John Wiley & Sons, Inc.
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  • 56
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    International Journal of Quantum Chemistry 49 (1994), S. 253-277 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This article addresses the chemical aspects of electronegativity: (1) What is its present status in the chemical community? (2) What are the necessary chemical criteria for a quantitative definition? (3) To what extent do contemporary proposals satisfy these criteria? (4) What connection can be made between the traditional free-atom scales and an in situ electronegativity appropriate for a particular atom in a specific molecule or solid? A longstanding special feature of electronegativity has been the seeming inability to measure it in the laboratory and this aspect proves to be a key to its definition. © 1994 John Wiley & Sons, Inc.
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  • 57
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    International Journal of Quantum Chemistry 49 (1994), S. 279-290 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Germanium chemical shifts were studied theoretically by the ab initio molecular orbital method. The compounds studied were GeMe4-xClx and GeMe4-xHx(x = 0-4). The calculated values of the germanium chemical shifts agreed well with the available experimental values. The germanium chemical shift is due to the p-electron mechanism that reflects the ligand electronic effect on the p-p* excitation term in the second-order paramagnetic term. For GeMe4-xHx, the chemical shift is almost linear to the number of the ligand, x. On the other hand, a U-shaped dependence is predicted for the chemical shifts of the GeMe4-xClx series and is shown to be caused by the strong and nonadditive electron-withdrawing ability of the Cl ligand. The diamagnetic contribution is relatively small for the chemical shift and is determined solely by a structural factor. © 1994 John Wiley & Sons, Inc.
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  • 58
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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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  • 59
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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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  • 60
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    International Journal of Quantum Chemistry 52 (1994), S. 993-1010 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We calculate the first ionization potential of atoms (3 ≤ Z ≤ 54) by means of a nonlocal density functional approach and the results are compared with those obtained using other recent density functional approaches based on density gradients. In these calculations, we use a nonlocal weighted spin-density approximation of exchange effects and a local spin-density approximation of Coulomb correlation, both based on novel forms of the pair-correlation functions. We also calculate the total energies of the He, Be, and Ne isoelectronic series. © 1994 John Wiley & Sons, Inc.
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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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  • 62
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    International Journal of Quantum Chemistry 49 (1994), S. 353-361 
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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 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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  • 63
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    International Journal of Quantum Chemistry 49 (1994), S. 343-352 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The effect of the truncation errors resulting from the numerical integration on the accuracy of the calculation results in the DV-Xα method is analyzed and it is shown that evaluating the overlap integrals analytically or adopting orthogonalized basis sets can reduce the errors on the density matrices to the second order. An approach to improve the calculation accuracy of the total energy is proposed based on the error analysis. The calculation results for several molecules are presented to demonstrate the conclusion. © 1994 John Wiley & Sons, Inc.
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  • 64
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    International Journal of Quantum Chemistry 49 (1994), S. 363-369 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Semiempirical MOPAC RHF/PM3 and BIRADICAL/PM3 calculations are performed on several selected analogs of the Neocarzinostatin chromophore. The critical distance parameter, heat of formation, and pertinent bond orders are reported for each model compound. Heats of reaction are also reported for the cyclization of several model compounds to their corresponding diradical products. It is suggested that 3.49 ÅR represents an upper bound to the critical distance in the cyclization of compound related to Neocarzinostatin chromophore. © 1994 John Wiley & Sons, Inc.
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  • 65
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    International Journal of Quantum Chemistry 52 (1994), S. 1051-1065 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: MS-Xα and SCCEH calculations on the Ag2+ complexes AgF64- and AgCl64- (displaying an elongated D4h symmetry) have been carried out for a better understanding of their experimental optical and EPR properties. As salient features, the present work supports that the unpaired electron in AgCl64- spends a little more time on ligands than on Ag2+, in agreement with the previous analysis of EPR and optical data for KCl:Ag2+. Furthermore, the five experimental optical transitions observed in that case are reasonably assigned. The first transition (observed at 12,500 cm-1) is assigned to a jump involving the 5a1g orbital built mainly (∽70%) from 3p orbitals of axial ligands, a fact that reflects the distinct level scheme for AgCl64- when compared to that for more ionic complexes. Calculations on AgF64- and AgF42- performed as a function of the equatorial Ag2+ -F- distance led to a reasonable understanding of experimental gyromagnetic and superhyperfine tensors displayed by Ag2+ in fluorides. The different relative decrease undergone by g‖- go (8%) and g ⊥ - go (28%) on passing from CsCdF3:Ag2+ to RbCdF3:Ag2+ is shown to be consistent with the formation of AgF64- and AgF42- complexes, respectively, related to the different substitutional position of Ag2+ in such lattices. The decrement of about 8.5% experienced by both g‖ - go and g⊥ - go values on going from CsCdF3:Ag2+ to NaF:Ag2+ is pointed out to reflect the different electrostatic potential (exerted by the rest of the lattice upon the complex) seen by AgF64- embedded in NaCl or perovskite-type lattices. © 1994 John Wiley & Sons, Inc.
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  • 66
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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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  • 67
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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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  • 68
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    International Journal of Quantum Chemistry 49 (1994), S. 539-548 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A set of exact conditions is compiled for the purpose of developing and testing approximations for the exchange-correlation energy as a functional of the electron density. Special emphasis is placed upon recently developed density-scaling relationships. Commonly used generalized gradient approximations are compared against several of these conditions. A direct tabular comparison of these functionals (not of calculated properties) with one another is also made. © 1994 John Wiley & Sons, Inc.
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  • 69
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    International Journal of Quantum Chemistry 49 (1994), S. 527-537 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have demonstrated molecular dynamics simulations using a combination of the classical molecular dynamics with density functional theory for argon clusters. Three different molecular dynamics schemes, which differ in their treatment of the potential energy and forces, have been carried out. The first uses a Lennard-Jones potential. In the second, the potential is computed using the Harris functional, and in the third, a combination of Lennard-Jones and Harris functional potentials is used. In addition to direct examination of the trajectories, the velocity autocorrelation function and its power spectrum have been computed to demonstrate the agreement between these three methods. The present studies show that a scheme that used a combination of model potentials and density functional theory provides a very useful tool for the dynamics simulation of systems that contain some fragments in which the analytical model potentials are not available. © 1994 John Wiley & Sons, Inc.
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  • 70
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    International Journal of Quantum Chemistry 49 (1994), S. 511-526 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Local spin density (LSD) methods were used to study the concerted 1,3-dipolar cycloadditions for fulminic acid plus acetylene, fulminic acid plus ethylene, and nitrone plus ethylene. Cartesian Gaussian double-zeta split-valence basis sets augmented with one set of polarization functions (DZVP) were used for the LSD calculations. The LSD calculations were performed with the LSD exchange functional (Dirac) and with the Vosko, Wilk, and Nusair correlation energy functional (VWN). Nonlocal spin-density corrections (NLSD) were estimated with the exchange functional of Becke and the correlation energy functional of Perdew (VWN + BP) and Becke, and the correlation energy functional of Lee, Yang, and Parr (B-LYP). Vibrational frequencies were computed at the VWN and B-LYP levels by numerical differentiation of the analytical first derivatives of the energy. Each of these reactions was examined using Hartree-Fock and Møller-Plesset perturbation theory for comparison. Geometry optimizations were carried out at the Hartree-Fock level with the 6-311G(d,p) basis set, and correlation energies were computed up to the MP4SDTQ/6-311G(d,p) level of theory. For the reactions of fulminic acid plus acetylene, fulminic acid plus ethylene, and nitrone plus ethylene, our best estimated density functional barrier heights are 7.8 ± 1.5, 8.9 ± 0.3, and 11.05 ± 1.9 kcal/mol, respectively. These results are in reasonable agreement with the correlated wave-function calculations and experimental estimates. © 1994 John Wiley & Sons, Inc.
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  • 71
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    International Journal of Quantum Chemistry 49 (1994), S. 549-557 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this work, 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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  • 72
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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
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    Topics: Chemistry and Pharmacology
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  • 73
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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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  • 74
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    International Journal of Quantum Chemistry 49 (1994), S. 559-573 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Use of orthogonalized Hartree product (OHP) orbitals as the reference orbitals in coupled-cluster (cc) calculations is presented. Since such orbitals are determined without exchange, they provide as “classical” a description as possible. The OHP orbitals were generated by implementing Harris's formulation of the original Hartree method. Some computational considerations of the formulation are discussed. A critical evaluation of the OHP method as an orbital localization scheme is presented. The OHP orbitals were used as the reference in CCD and CCSD calculations and compared with corresponding Hartree-Fock (HF) reference CC results. The average variation of localized Hartree product (LHP) reference CCSD energy from that of the HF reference is 0.83 kcal/mol, whereas for CCD, the average variation is 234 kcal/mol, indicating the importance of single-excitation effects in CC calculations with non-HF references. © 1994 John Wiley & Sons, Inc.
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  • 75
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    International Journal of Quantum Chemistry 52 (1994), S. 1273-1285 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method is proposed how to calculate the correct density matrix of an infinite polymeric chain from that of a standard finite supercell calculation. The density matrix of the finite supercell is transformed into k-space for all k-values allowed by the periodic boundary conditions. The k-dependent matrices are then unitarily transformed, with each unitary matrix being represented by a set of complex rotation matrices. It is shown that the corresponding angles can be interpolated and extrapolated toward the zone boundaries in a straghtforward manner and that this extrapolation can be done from any finite supercell with reasonable accuracy. This gives rise to an infinite system density matrix for which all fundamental properties are guaranteed by construction. This infinite system density matrix may be used to construct a corrected density matrix for the finite supercell calculation. © 1994 John Wiley & Sons, Inc.
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  • 76
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    International Journal of Quantum Chemistry 52 (1994), S. 1287-1299 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Calculations of finite atomic clusters based on the Hartree-Fock self-consistent field theory are modified to model more closely the ideal behavior of the infinite system. The density matrix of the standard finite supercell calculation is extrapolated to an infinite supercell so that it contains information from a continuum of k points in the first Brillouin zone. This modification is incorporated into the self-consistency loop of the MOPAC quantum chemistry program and leads to improved results compared to a standard finite supercell calculation. Heats of formation, bond lenghts, and electronic properties converge more quickly to the correct ground-state values. For polyacetylene, we obtain a reduced bond-length alternation of Δr = 0.084 Å, which is in agreement with more sophisticated calculations containing electron correlation effects. © 1994 John Wiley & Sons, Inc.
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  • 77
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    International Journal of Quantum Chemistry 52 (1994), S. 1317-1328 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Photoenolization of a ketone in its lowest triplet state was examined using the AM1 semiempirical molecular orbital method with complete geometry optimization at the configuration interaction level in the restricted Hartree-Fock frame. The theoretical barrier fits with a polynomial given by Eq. (1) and the probability of tunneling of hydrogen was calculated by the WKB method. The density of states above the zero-point level was estimated by the Whitten-Rabinovitch approximation. Rate constants for the hydrogen and deuterium-abstraction processes via tunneling were then computed at different temperatures. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 52 (1994), S. 1301-1316 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This article deals with a dynamical system that generalizes the Kepler-Coulomb system and the Hartmann system. It is shown that the Schrödinger equation for this generalized Kepler-Coulomb system can be separated in prolate spheroidal coordinates. The coefficients of the interbasis expansions between three bases (spherical, parabolic, and spheroidal) are studied in detail. It is found that the coefficients for the expansion of the parabolic basis in terms of the spherical basis, and vica versa, can be expressed through the Clebsch-Gorden coefficients for the group SU(2) analytically continued to real values of their arguments. The coefficients for the expansions of the spheroidal basis in terms of the spherical and parabolic bases are proved to satisfy three-term recursion relations. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 52 (1994), S. 1-2 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 80
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    International Journal of Quantum Chemistry 52 (1994), S. 17-31 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new class of calcium antagonists (dibenzotricyclic compounds) is studied by means of reaction field ab initio calculations and molecular dynamics simulations. The central ring of these tricyclic molecules is found to be more important to the calcium antagonistic potency than the two phenyl rings. The central ring with antagonistic potency shows hydrophobic character, thus the interaction between the drug and the binding sites is assumed to be dominated by hydrophobic interactions. Variation of the flexure angle, the angle between the two phenyl rings, does not change the hydrophobic property of the central ring significantly, therefore it is not expected to affect the interaction between the drug and binding site directly. The effect of the flexure angle on calcium antagonistic potency, the relation between drug affinity of these tricyclic molecules and their ionization energies, and the interaction of calcium ions with the central ring are discussed. © 1994 John Wiley & Sons, Inc.
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  • 81
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    International Journal of Quantum Chemistry 52 (1994), S. 3-15 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The previously demonstrated capacity to utilize the hydrophobic folding and assembly transition in designed model proteins to perform diverse energy conversions is formalized in terms of three postulates with an associated 15 corollaries. Each corollary defines one of the 15 pairwise free-energy transductions involving the six intensive variables: mechanical force, temperature, pressure, chemical potential, electrochemical potential, and electromagnetic radiation. The first postulate directly involves the input of thermal energy to raise the temperature from below to above that temperature required to drive hydrophobic folding and assembly, with the resultant capacity to perform useful mechanical work. The second postulate considers the energy inputs that can lower or raise the temperature range over which the hydrophobic folding transition occurs; these energy inputs can thereby perform mechanical work. The third postulate treats the energy conversions not involving mechanical force, whereby a pair of functional groups becomes coupled by each being a part of the same hydrophobic association process with each functional group being able individually to drive the hydrophobic folding transition and thereby to change the state of the second functional group in an energy-conversion process. It is then shown how a model protein can be so designed as to function in a second-order process treated by Postulate III and how the efficiency of energy conversion can be enhanced. Finally, these energy-conversion studies using the hydrophobic folding and assembly transition in model proteins are related to a theoretical model for cooperative hydrophobic folding and to experimental studies suggesting a central role for hydrophobic folding in the general problem of protein folding. © 1994 John Wiley & Sons, Inc.
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  • 82
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    International Journal of Quantum Chemistry 52 (1994), S. 195-203 
    ISSN: 0020-7608
    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 complete coupled-cluster singles, doubles, and triples (CCSDT) calculations have been performed with Hartree-Fock (HF) and Brueckner (B) orbitals. Calculations have been performed with a double-zeta plus polarization basis set on the H2O, SiH2, NH2, BeO, C2, CN+, and BN molecules. Calculations on H2O and SiH2 at equilibrium and stretched geometries show negligible difference between HF-and B-CCSDT energies. This is also true for NH2, except when the bonds have been stretched to twice their equilibrium values, at which point there is about a 2.5 milli-Hartree (mEh) difference. Calculations on the isoelectronic systems BeO, C2, CN+, and BN were performed at equilibrium geometries. Even though these systems have large T1 amplitudes, the difference between HF- and B-CCSDT energies is only about 1 mEh. For the CCSD method and the CCSD(T) method, which includes triple excitations in an approximate, noniterative manner, however, somewhat larger differences are observed between and HF-and B-CC results. Finally, some properties of BN were computed using HF- and B-CC methods. There are quite small differences between the HF- and B-CCSDT results, but significantly larger ones for the more approximate CCSD and CCSD(T) methods. For this difficult system, where the CCSD(T) approximation seems to be inadequate for HF orbitals, the use of Brueckner orbitals improves the agreement of CCSD(T) with CCSD(T) substantially for re and we, although the difference for μ is unaffected. © 1994 John Wiley & Sons, Inc.
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  • 83
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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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    International Journal of Quantum Chemistry 52 (1994), S. 205-214 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Relativistic pair correlation energies of Xe were computed by employing a recently developed relativistic coupled cluster theory based on the no-pair Dirac-Coulomb-Breit Hamiltonian. The matrix Dirac-Fock-Breit SCF and relativistic coupled cluster calculations were performed by means of expansion in basis sets of well-tempered Gaussian spinors. A detailed study of the pair correlation energies in Xe is performed, in order to investigate the effects of the low-frequency Breit interaction on the correlation energies of Xe. Nonadditivity of correlation and relativistic (particularly Breit) effects is discussed. © 1994 John Wiley & Sons, Inc.
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  • 85
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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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    International Journal of Quantum Chemistry 52 (1994), S. 237-251 
    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
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  • 87
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    International Journal of Quantum Chemistry 52 (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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  • 88
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    International Journal of Quantum Chemistry 52 (1994), S. 1-2 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 89
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    International Journal of Quantum Chemistry 52 (1994), S. 3-5 
    ISSN: 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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  • 90
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    International Journal of Quantum Chemistry 52 (1994), S. 133-156 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A theoretically contracted agonist conformation of potent phenoxypropanolamine derivatives on the β1-adrenoceptor has been analyzed in detail. The main effect of the enthalpic contraction of some 6.0-7.0 kcal/mol arises from the movement of the nitrogen atom toward the aromatic ring by 0.7-0.8 Å, requiring some 3.0-3.5 kcal/mol. A second effect arising from the contraction can be a dihedral rotation of some 30° around the O—CH2 bond of the planar anisole moiety. This rotation is correlated with an effect arising from “in-plane” deformation of the anisole moiety where opening of the relevant bond angle releases steric constraints for this rotation. Ortho-substituents assist this rotation indirectly through hyperconjugation with the lone pair of the OCH2 group, electron-attracting substituents opening this bond angle and lowering the energy required to reach a given bond-angle deformation. The adjacent ring meta-substituent can be similarly affected, the strength of the total effect being also of the order of 3.0-3.5 kcal/mol. The net effect gives rise to a further contraction of the nitrogen atom and the betahydroxyl group toward the aromatic ring, the beta-hydroxyl group showing a contraction of up to 0.4-0.5 Å along the main axis of the conformer. The deformed conformation is consistent with the predicted conformer of a fixed-ring benzdioxepine molecule that possesses the highest degree of partial agonism within the set of phenoxypropanolamine agents. It is concluded that ortho-substituents in phenoxypropanolamine derivatives can retain steric freedom in both agonist and antagonist action provided that the substituent can accommodate the required deformation, both agonist and antagonist conformer forms lying within one unbound conformation. The agonist conformer is consistent with the proposed model for the ligand-activated transmembrane proton transfer in the β1-adrenoceptor where a contraction along the main axis of the ligand conformer (with some attendant distortion in the position of the β-hydroxyl moiety) is required to activate proton transfer. © 1994 John Wiley & Sons, Inc.
    Additional Material: 8 Ill.
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  • 91
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    International Journal of Quantum Chemistry 52 (1994), S. 7-10 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Developments, through some 35 years, in the theory of electronic structure and reaction dynamics are reviewed from the perspective available to a close collaborator to Yngve Öhrn. © 1994 John Wiley & Sons, Inc.
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  • 92
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    International Journal of Quantum Chemistry 52 (1994), S. 261-264 
    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: Similarities between the Franck-Condon effect and the concept of Squeezed States have been advanced as a conjecture in the past, or noticed as a marginal fact, without giving a full explanation of how these two important concepts are connected. In this work we outline a unifying formulation that contains as special cases both phenomena. The power of the new approach is shown in a general formula from which all particular cases can be derived. © 1994 John Wiley & Sons, Inc.
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  • 93
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    International Journal of Quantum Chemistry 52 (1994), S. 265-270 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The contribution of the electron correlation effects to the calculated dipole polarizability and hyper-polarizability of the first-row atomic anions is calculated and analyzed. It is shown that the total correlation contribution to the dipole hyperpolarizability is extremely large with the Hartree-Fock model accounting for only a small fraction of the accurate result. The linear and, more pronounced, the nonlinear response of atomic anions to the application of an electric field emphatically shows the effects of the correlated motion of the electrons. © 1994 John Wiley & Sons, Inc.
    Additional Material: 3 Tab.
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  • 94
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    International Journal of Quantum Chemistry 52 (1994), S. 271-282 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Previous measurements of tunnel conductance in heterostructures and experiments with Josephson junctions suggest quantum tunneling has a definite duration. We use semiclassical methods to determine the effects of this delay on the tunneling current in a laser-assisted STM. A planar-planar STM model is used with the exact multiple image potential, and the energy distribution for a free-electron metal. It is necessary to average over the phase at barrier entry, and iteration with back propagated solutions is required to obtain the transmission coefficients for evenly spaced phases and specified energies at barrier entry. The simulations suggest that the dependence of the tunneling current on the wavelength of illumination can serve as a basis for determining the duration of barrier traversal. A power flux density of 1011 W/m2 would be required at several wavelengths from 1 to 10 μm. It is possible that thermal effects could be separated from the modeled phenomena by determining the time dependence of the tunneling current with a pulsed laser. © 1994 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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  • 95
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new approach for solving multielectronic integrals in quantum chemistry is presented. The Coulomb potential r12-1 is expanded in a complete function set within a finite spherical domain of radius R. This expansion yields a series where the \documentclass{article}\pagestyle{empty}\begin{document}$ \vec r_1 {\rm and\, }\vec r_2 $\end{document} variables are symmetrically separated. The generalization to other potentials is briefly discussed. The Yukawa potential exp(-ζr12)/r12, and the functions exp(-ζr122) and exp(-ζr12) are considered as particular cases. As an example of the potentialities of this approach, a certain type of four-electron integrals that appear in the Hylleraas CI method are evaluated. Results thus obtained are discussed in terms of the approximations made and compared with values obtained using other procedures available in the literature. © 1994 John Wiley & Sons, Inc.
    Additional Material: 1 Tab.
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  • 96
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    International Journal of Quantum Chemistry 52 (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: We present the effects of structure, chemical binding, and number of layers on stopping for clean, unreconstructed diamond (100) one- to four-layer films with lattice constants fixed at crystalline values, and for graphite mono- and dilayers at their optimized bond lengths. First Born approximation stopping cross sections for protons incident normally on the films were obtained from orbital versions of the kinetic theory of stopping and of the local plasma approximation. The required electron momentum density was generated from the real space Kohn-Sham orbitals. We find a strong static quantum size effect in the stopping of diamond ultra-thin films as opposed to graphite, for which the stopping is only very weakly dependent on the layer number. © 1994 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 97
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    International Journal of Quantum Chemistry 52 (1994), S. 309-314 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A comparison is made between the ionization potentials (IPS) calculated by the extended Koopmans' theorem (EKT) and by taking energy differences (ΔCI) from configuration interaction calculations in the same basis. Several ionization potentials were calculated for LiH, He2, and Li2. The best ΔIP, the difference between the EKT IP and the corresponding ΔCI value, was 0.05 meV for the 2σ orbital for LiH and 83.5 meV for the 3σ orbital. The ΔIPS for He2 were 0.7 meV for the 1σu orbital, 6 eV for the 2σu orbital, 5 meV for the 2σg orbital, and 3 eV for the 3σg orbital. The ΔIPS for Li2 are 0.1 meV for 2σg, 53 meV for 3σg, 0.6 meV for 2σu, and 1.73 eV for 3σu. © 1994 John Wiley & Sons, Inc.
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  • 98
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    International Journal of Quantum Chemistry 52 (1994), S. 129-137 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular dynamics simulations are used to study hypervelocity impacts of an ultrathin flyer plate with a semi-infinite two-dimensional model diatomic molecular solid. We show that these hypervelocity impacts can produce a dissociative phase transition from a molecular to a close-packed solid in the target material. We also show that hypervelocity impacts of ultrathin plates can produce extensive chemical reactions leading to a detonation accompanied by a phase transition in an energetic version of the model. © 1994 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 99
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    International Journal of Quantum Chemistry 52 (1994), S. 113-127 
    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: Optical nonlinearities affecting absorption spectra, above threshold dissociation, bond softening, and vibrational trapping are among the most important processes that have so far been examined when describing the dynamics of multiphoton excitation and fragmentation of molecular systems subject to intense laser fields. The relative merits of time-dependent and independent approaches are overviewed and discussed within the framework of H2+ photodissociation for which recent experimental data are available. Isotopes H2+ - D2+ behave in a markedly different manner with respect to the radiative coupling. Some of these differences are exploited for the detailed understanding of the underlying mechanism of the bond softening process. It is shown that this is not merely a single photon potential barrier lowering mechanism as has previously been invoked, but a competition between this mechanism and another five photon exchange mechanism, namely the simultaneous absorption of three photons followed by the subsequent emission of two photons. As for the vibrational trapping process, the relative sensitivity of the isotopes to the laser characteristics is such that, by adequately adjusting the wavelength and the intensity, one can suppress the fragmentation of one of the isotopes by locally confining it, whereas the other dissociates very fast. This is used as a possible scheme for isotope separation using intense lasers. © 1994 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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  • 100
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    International Journal of Quantum Chemistry 52 (1994), S. 139-154 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Relying on theoretical developments exploiting quasispin and the pseudo-orthogonal group in the Hubbard model of cyclic polyenes, the general expressions for generating polynomials, providing the dimensional information for relevant irreducible representations, were derived (M.D. Gould, J. Paldus, and J. Cízek, Int. J. Quantum Chem., in press). These generating polynomials result from q-dimensional formulas through rather tedious algebraic manipulations involving ratios of polynomials with fractional powers. It is shown that these expressions may be efficiently handled using the symbolic manipulation language MAPLE and the dimensional information for an arbitrary spin, isospin, and quasimomentum obtained. Exploitation of symbolic computation for other group theoretical problems that are relevant in quantum chemical calculations and their relationship with Gaussian polynomial based combinatorial approaches is also briefly addressed and various possible applications outlined. © 1994 John Wiley & Sons, Inc.
    Additional Material: 4 Tab.
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