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  • 1990-1994  (2,366)
  • Computational Chemistry and Molecular Modeling  (2,365)
  • Nuclear reactions
  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 309-320 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that for a number of molecules there exists a Hammett-like equation for the chemical hardness or the HOMO-LUMO energy gap. Substituent effects can be factored out of the molecular hardness. Preliminary results show that a hardness substituent constant, similar to the Hammett σ-constant, can be defined and determined for each substituent. © 1994 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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.
    Additional Material: 8 Tab.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 3-10 
    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: The geometries of the reactant, product, and transition state of the reaction between acetylene and the hydrogen atom have been optimized at the level of HF/6-31G using the energy gradient method. The barrier height for this reaction was calculated to be 105.9 kJ/mol with a zero-point energy correction and elimination of spin contamination. The intrinsic reaction coordinate (IRC) for this reaction was traced and the coupling between the IRC and normal modes was analyzed along the IRC. The variational transition-state theory with the correction of the tunnel effect has been used to evaluate the reaction rate constant. It is in good agreement with an experimental estimate. © 1994 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 59-61 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 35-43 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The recently introduced second-order similarity index gAB was generalized by using the geminal and/or spingeminal expansion of pair densities that they are derived from. The comparison with previous related studies confirms that the generalization does increase the information content of new indices, especially in providing the information about the difference in the spin recoupling of pure singlet and triplet states of electron pairs. Analogously, as in previous studies, the approach was applied to the analysis of several selected pericyclic reactions and possible mechanistic implications arising from the increased information content are briefly discussed. © 1994 John Wiley & Sons, Inc.
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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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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  • 9
    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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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 133-134 
    ISSN: 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. 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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  • 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. 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.
    Additional Material: 3 Ill.
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  • 14
    Electronic Resource
    Electronic Resource
    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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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 353-367 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Single-reference coupled-cluster methods corresponding to various approximate forms of the cluster operator including up to four-electron operators are applied to the simple H4 model system with the aim of numerically studying the attainability and properties of multiple solutions of the systems of nonlinear equations for the cluster amplitudes. Various solutions have been obtained for the excited states 21 A1 and 51 A1. It is documented that there exist nonphysical nonstandard solutions that yield energies very close to the physical ones. A simple way of selecting the physically meaningful solutions is proposed. It is demonstrated that the attainability of nonstandard solutions describing a state of H4 within the framework of a given SE-CC method strongly depends on the cluster structure of that state. © 1994 John Wiley & Sons, Inc.
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  • 18
    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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  • 19
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 395-400 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We apply the HH-GLF method, a new simple hyperspherical harmonic method proposed recently by one of us, to directly solve the three-body Schrödinger equation for e+e-e+. Uniformally convergent energy eigenvalues are obtained with only several GLF and the obtained ground-state energy with 200 HH and 6 GLF is -0.26124 au, which is very near the exact value of -0.26200 au. Energy results for maximum global momentum Km ≤ 20 are compared with those from some other hyperspherical techniques carefully, and we find that, in the example of e+e-e+, the HH-GLF method can yield results as accurate as the best available other HH method, but is conceptually simpler and more convenient for practical calculations with a large number of hyperspherical harmonics. © 1994 John Wiley & Sons, Inc.
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 401-410 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The problems connected with the simulation by the diffusion quantum Monte Carlo method of atomic and molecular systems in the two-dimensional (2D) space have been investigated on the 1 2S and 2 2S states of the hydrogen atom and on the 1 1S and 1 3S states of the helium atom, assuming r-1 Coulomb interactions between the particles. The potential surface of the 1Σg+ state of the 2D hydrogen molecule has been calculated in the range 0.1-1.5 bohr: A dissociation energy of 1.2703 Hartrees is found at the equilibrium distance of 0.3639 bohr. © 1994 John Wiley & Sons, Inc.
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  • 21
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 581-589 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A diatomic charge density distribution function may be partitioned into free atom and delocalization components. Integrating the electrostatic force on one of the nuclei as the atoms join to bond yields a repulsive atom superposition energy Er(R) and an attractive delocalization energy Ed(R) which, when added, equal the Born-Oppenheimer potential energy, E(R). Bond stretch force constants may be calculated to quite good accuracy from the Poisson equation ▽R2E(R) = 4πZbpa(r = R), where Zb is the charge of nucleus b at a distance of R from nucleus a and pa is the charge-density distribution function of isolated atom a. This equation follows if the delocalization density component is rigid during molecular vibrations and Ed = c/R, where c is a constant. Ed(R) is well approximated by the change in orbital energy, δEMO(R), for the bond-forming reaction a + b → a - b as obtained from a modified extended Hückel procedure. The resulting E() = Er(R) + δEMO(R) generalizes immediately to polyatomic molecules and solids and can be used for the calculation of structures and other properties and their molecular orbital interpretation. The current understanding of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory is presented in this article. It is pointed out that a parallel exists with the density functional theory, wherein the Born-Oppenheimer potential function may be constructed from energy points E(R) that are functionals of the molecular charge density distribution function. Calculations of molecular structures with density functional theory has also become possible only through the introduction of electron orbitals. However, the approaches are otherwise different, for density functional theory works with the total energy of the molecule and the ASED approach works with the molecular binding energy obtained by integrating the electrostatic force on a nucleus, which is zero for the isolated atoms. © 1994 John Wiley & Sons, Inc.
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  • 22
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994), S. 87-97 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A formulation for calculating frequency-dependent hyperpolarizabilities in the coupled-cluster (CC) theory with the Brueckner orbitals is presented. Our quasi-energy derivative (QED) method is applied, and explicit expressions for response properties up to third order [μ, α(-ω1;ω1), β(-ωσ;ω1, ω2)] are derived. The position of the resonances is determined from the RPA-like equation. It is found that the pole structures are not compatible with ones of the exact response functions. An extension of the coupled Brillouin-Brueckner CEPA-0 by Kutzelnigg to the time-dependent case is also discussed. © 1994 John Wiley & Sons, Inc.
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  • 23
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 151-160 
    ISSN: 0020-7608
    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 Hartree-Fock, Møller-Plesset perturbation theory (MP2), and quadratic configuration interaction, using single and double substitutions (QCISD), calculations were carried out for the NF3+ ion. Optimized structures were examined at the various levels of theory. Calculation of the inversion barrier height shows the importance of optimizing the geometry at the post-Hartree-Fock level and the inclusion of polarization functions. The best calculated inversion barrier was 13.3 kcal/mol, compared to an experimental value of 17.3 kcal/mol. The dissociation transition state was computed to determine the well depth of the NF3+ ion and its stability toward dissociation. The computed well depth was 28 and 48 kcal/mol at the SCF and MP2 levels, respectively. © 1994 John Wiley & Sons, Inc.
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  • 24
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 51 (1994), S. 407-415 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An application of symplectic implicit Runge-Kutta (RK) integration schemes, the s-stage Gauss-Legendre Runge-Kutta (GLRK) methods of order 2s, for the numerical solution of molecular dynamics (MD) equation is described. The two-stage fourth-order GLRK method, the implicit midpoint rule, and the three-stage diagonally implicit RK method of order four are studied. The fixed-point iteraction was used for solving the resulting nonlinear system of equations. The algorithms were applied to a complex system of N particles interacting through a Lennard-Jones potential. The proposed symplectic methods for MD integration permit a wide range of time steps, are highly accurate and stable, and are thus suitable for the MD integration. © 1994 John Wiley & Sons, Inc.
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  • 25
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 333-351 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The method of electron nuclear dynamics (END) developed by Öhrn and Deumens has been applied to the coupling of electronic structure and vibrational motion in the hydrogen molecule with the aim of elucidating and illustrating the characteristic features of this method. © 1994 John Wiley & Sons, Inc.
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  • 26
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 369-384 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Recursive formulas are derived for generating the conjugated-circuit polynomials for circuits up to size n = 6, 10, or 14 on regular chain polyhex graphs. These counting polynomials are of use for a simple valence-bond resonance-theoretic derived model: the conjugated-circuit model. © 1994 John Wiley & Sons, Inc.
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  • 27
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 385-394 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The relative stability of the four tautomeric forms of α-hydroxytetronic acid was calculated with full geometry optimization at the STO-3G, 3-21G, 6-31G*, and 6-31G** SCF levels. Correlation effects were estimated using the MP2 method. Intramolecular hydrogen bonds are found to be of primary importance for the relative stability of the various tautomers, and the same tautomer that in the case of L-ascorbic acid is experimentally observed in the crystal as well as in solution is found to be the most stable one, if polarization functions on the hydrogen atoms are taken into account. It is concluded that even in the gas phase this tautomer predominates in α-hydroxytetronic acid as well as in L-ascorbic acid. © 1994 John Wiley & Sons, Inc.
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  • 28
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 50 (1994), S. 411-428 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The quantum defect orbital (QDO) method and its relativistic (RQDO) counterpart have been employed in this work to compute oscillator strengths for several fine-structure transitions in members of the copper isoelectronic sequence up to Z = 92. The RQDO results are found to be in quite good agreement with the best estimates derived from other sources, whenever the comparison is possible. © 1994 John Wiley & Sons, Inc.
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  • 29
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    International Journal of Quantum Chemistry 51 (1994), S. 569-575 
    ISSN: 0020-7608
    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 for finding the chemical potential for an electronic system with density ρ = Σρi represented within the Kohn-Sham approximation is proposed. To find the chemical potential of the system under consideration, we propose to refer to the definition μ = δE/δρ and to apply the mathematical properties of functional derivatives. Particularly, in the case examined, the result μ = μ(r) ≠ const has been obtained, which may be explained in the framework of the calculus of variation. Taking the limit limr→∞ μ(r) as the best approximation to the proper equilibrium chemical potential of a free atom, one obtains μ = -I, where I denotes first ionization energy. A possibility of further applications of the proposed method in relation to crystalline systems is also discussed. © 1994 John Wiley & Sons, Inc.
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  • 30
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 52 (1994), S. 1-8 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The molecules of M2Se and M2I+ (M = Ag, Au) are used as test models to study the quality of the pseudopotential methodology when using the Hartree-Fock and second-order Møller-Plesset methods. We have used two implementations of the Wadt and Hay pseudopotentials: the new large- and small-core pseudopotentials (known as LANL 1DZ and LANL2DZ in GAUSSIAN-92) and the traditional one used for a long time. It is found that the optimum geometry depends on the type of pseudopotential selected. A comparison with the results computed when all the electrons are taken explicitly into account shows that the new LANL1DZ and LANL2DZ give the right answer. Using these potentials, the geometry and potential energy surface of the M2Se and M2I+ systems is explored and its optimum geometry found. © 1994 John Wiley & Sons, Inc.
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  • 31
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    International Journal of Quantum Chemistry 51 (1994), S. 13-25 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Full configuration interaction (FCI) calculations are reported for the closed-shell cyclic polyenes CNHN (N = 6, 10, 14, 18) in the Pariser-Parr-Pople (PPP) approximation, as a function of the hopping parameter β. A wide range of values of β is considered, from a highly correlated situation, β = 0, to a very weakly correlated limit, β = -10 eV. An estimate of the role of higher than two-body connected cluster components was done, through a partial (i.e., limited to two-body terms in the exponent) cluster analysis performed on the FCI wave function. The comparison with the approximate coupled pair theory that accounts for quadruply excited clusters [see P. Piecuch and J. Paldus, Theor. Chim. Acta 78, 65 (1990)] shows a good agreement in the whole range of the hopping parameter, particularly when the contribution of connected triples excitations is also taken into account. © 1994 John Wiley & Sons, Inc.
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  • 32
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    International Journal of Quantum Chemistry 52 (1994), S. 59-70 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We studied the possibility of defining a one-electron effective Hamiltonian in the form of a Hückel-type matrix, taking into account the effect of the environment of the atoms in different molecules (the E2HT). It is found that the appropriate basis set in which this Hamiltonian is defined can be determined by using diatomic fragments. Concerning the matrix elements themselves, the appropriate information can be taken in fragments with a size similar to that of ethane. © 1994 John Wiley & Sons, Inc.
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  • 33
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    International Journal of Quantum Chemistry 52 (1994), S. 89-96 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We presnet a theoretical study of the effect of the conjugation length on the electronic properties and second-order molecular polarizabilities β in p-amino-p′-nitrodiphenylacetylene molecules where the number of triple bonds in the conjugated segment varies from 1 to 4. The β values are calculated via an intermediate neglect of differential overlap/single configuration interaction (INDO/SCI) sum-over-states (SOS) approach. We test the convergence of the SOS method and the validity of the two-state model to describe the β response. The results indicate that increasing the conjugation length results in a decrease of the charge transfer within the molecule. The two-state model is shown to break down as the conjugated segment extends to four triple bonds; this is due to the appearance of several low-lying nearly isoenergetic excited states that significantly contribute to the β response. The theoretical results are in excellent agreement with recent experimental data. © 1994 John Wiley & Sons, Inc.
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  • 34
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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: Based on the generalized relationship for calculating the nuclear spin-spin coupling constants and the correlation of the bond stretching frequencies with the coupling constants, a novel generalized reationship, which includes the contributions of not only the hybrid orbitals, but also the net atomic charges, is introduced for calculation of the bond stretching frequencies and employed to elucidate the C—H stretching frequencies in hydrocarbons and heterosubstituted hydrocarbons on the basis of the MBOHO calculation employing the CNDO/2 approximation. By use of the obtained concrete realtionships, one can get different νCH value for the C—H bonds existing in different chemical environments, which is coincident with chemical intuition. The calculated numerical results show that for hydrocarbons the contribution of the net atomic charges can be neglected, but it is necessary for heterosubstituted hydrocarbons to include the contribution of the net atomic charges to the C—H stretching frequencies. The calculated C—H stretching frequencies are in good ageement with the experimental data, which shows its reasonableness. © 1994 John Wiley & Sons, Inc.
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  • 35
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    International Journal of Quantum Chemistry 52 (1994), S. 127-133 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio self-consistent field (SCF) calculations are performed with the standard 6-31G* basis set for all-trans conjugated oligomers C2n+2H2n+4. The canonical occupied and virtual molecular orbitals (MOs) are separately localized by unitary transformations. Due to the localization, the perturbation operator is partitioned into two-particle and into single-particle terms; the MBPT is, therefore, a double-perturbation expansion in this case. By using the localized representation of the MBPT, the correlation energy contributions can be partitioned into local and nonlocal effects. It can be shown that the local effects are very important and well transferable, which makes possible the calculation of the correlation energy of larger molecules if the localized molecular orbitals (occupied and virtual) of smaller related molecules are known. © 1994 John Wiley & Sons, Inc.
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  • 36
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    International Journal of Quantum Chemistry 52 (1994), S. 147-152 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An iso-energy cutoff scheme is introduced for the calculation of the potential of mean force between two ions in water. The cutoff criterion is based on the optimal interaction of the water dipole with the ion pair, for which analytical expressions are derived. Formulas are also derived to characterize the solvent reorganization contribution to the potential of mean force. Treatment of the contributions from waters outside the cutoff is also discussed. © 1994 John Wiley & Sons, Inc.
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  • 37
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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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  • 38
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    International Journal of Quantum Chemistry 52 (1994), S. 191-197 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantioselective hydrogenation of α-ketoesters with cinchona-modified platinum catalysts has been studied theoretically in order to rationalize the interaction between the chiral modifier and the substrate. The structure of the probable transition complex has been calculated for the system methyl pyruvate (substrate)-cinchonidine (modifier) using quantum chemistry techniques at both ab initio and semiempirical levels and molecular mechanics. The calculations indicate that crucial interaction occurs via hydrogen bonding of the quinuclidine nitrogen and the oxygen of the α-carbonyl moiety of methyl pyruvate and confirm earlier experimental evidence that the quinuclidine nitrogen of the cinchonidine is involved in the interaction leading to enantiodifferentiation. The resulting complex, which resembles to a half-hydrogenated state of the substrate, is shown to be a possible transition state for the enantioselective hydrogenation of α-ketoesters. © 1994 John Wiley & Sons, Inc.
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  • 39
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    International Journal of Quantum Chemistry 52 (1994), S. 211-225 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Coupled-cluster (CC) methods at the level of CCSD, CCSD+T(CCSD), CCSD(T), CCSDT-1, and CCSDT-3 are applied to calculations of the dipole moment and polarizability of the CN molecule, ionization potentials and electron affinities of the oxygen and iron atoms and CN molecule, and the energy splitting of the 5D and 5F states of the iron atom. Both UHF and ROHF references are applied. Extended basis sets are used in some comparison of CC data to experiment. All calculated atomic and molecular properties are known as challenging problems, suitable for a careful analysis of the performance of sophisticated versions of the CC approach. Attention is paid to energy terms distinguishing CCSD(T) from CCSD+T(CCSD). We exploit results from various iterative and noniterative high-level CC methods in the assessment of error bars in calculations of atomic and molecular properties. © 1994 John Wiley & Sons, Inc.
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  • 40
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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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  • 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. 295-299 
    ISSN: 0020-7608
    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 quantum chemical definition is proposed of the full atomic valence, VA, taking into account both the covalent and ionic parts of the chemical bonds formed by atoms in molecules and crystals. The full atomic valencies, covalencies, and charges on atoms are calculated for nickel-oxygen crystalline compounds in the CNDO band theory approximation. A comparison of the chemical bonding in nickel and copper crystalline oxides is given. © 1994 John Wiley & Sons, Inc.
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  • 43
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    International Journal of Quantum Chemistry 52 (1994), S. 321-328 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method of description of a crystalline solid is presented and discussed in the light of the variational method. The proposed method refers to the muffin-tin cellular method but it is formulated in the language of electron density, without referring to wave-function treatment. Model calculations of p-dependent binding energy for alkaline metals have been performed. The results show that the proper adjustment of the effective charge of atomic core zeff parametrizing the electron density in the cell allows one to reproduce exact values of binding energy at the equilibrium distance. The numerical results prove that the dependence between the charge zeff and the radius of the cell RMT may be represented by the functión zeff = (21)1/2(1 + B exp(-ARMT)), where A and B are numerical parameters changing with high regularity throughout the period and I denotes the first ionization energy. © 1994 John Wiley & Sons, Inc.
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  • 44
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    International Journal of Quantum Chemistry 52 (1994), S. 365-373 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The periodic ab initio Hartree-Fock linear combination of atomic orbitals program CRYSTAL has been used to investigate the electronic structure of the wurtzite-type phase of SiC. Binding energy, lattice parameters (a,c), and the internal cordinate have been optimized. Inclusion of polarization functions, which has been shown to be necessary in the description of related systems, has been used to compute the bulk modulus and its pressure derivative and the central zone A1 transverse optical phonon frequency. The electronic structure of SiC is analyzed using density of states projections and electron density maps. Results are compared with those obtained for the cubic zinc blende-type phase. © 1994 John Wiley & Sons, Inc.
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  • 45
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    International Journal of Quantum Chemistry 52 (1994), S. 353-364 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Geometry optimizations on molecular clusters of the five-membered heterocyclic molecules pyrrole, furane, thiophene, and selenophene were carefully carried out. It is shown that the value of the band gap cannot be related to the bond alternation in the aromatic compounds. However, a correlation of the gap value with the ionization potential of the heteroatom in the ring is found and can be explained by the different stabilizations of the LUMO energy of the heterocycle with respect to the butadiene backbone. The energy band structure including also corrections for the electron correlation effects is reported. The newly developed variational method for the calculation of static polarizabilities of polymers is reviewed and applications to cyclopropene and heteroaromatic derivatives are reported. Finally, a possible approach to treat dynamical polarizabilities of polymers based on perturbation theory is briefly outlined. © 1994 John Wiley & Sons, Inc.
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  • 46
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    International Journal of Quantum Chemistry 52 (1994), S. 413-423 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The many-electron system in one and two dimensions are studied within the geminal approach. The analytical expressions for the wave functions and ground-state energies are obtained for a number of 1-D and 2-D systems: conjugated polymers, organic conductors, 2-D conductors with square lattices, and others. It is shown that electron excitations of a kink type can exist in 2-D systems with mixed valency. In this case, the correlation pairing of current carriers arises as a result of correlation effects leading to superconducting properties of the system. © 1994 John Wiley & Sons, Inc.
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  • 47
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    International Journal of Quantum Chemistry 51 (1994), S. 447-463 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The convergence properties of the expansions of (a) the function 1/r and (b) the function exp(-αr) in an even-tempered basis of Gaussians are studied analytically. The starting points are the Gaussian integral representations of 1/r and exp(-αr). One arrives at an expansion in a finite number of Gaussians in three steps: (1) a restriction of the integration domain, (2) a variable transformation, and (3) discretization of the integral. The cutoff error goes in both cases essentially as exp(-ah), and the discretization error, as exp(-b/h). The minimum overall error is reached for the β-parameter of an even-tempered basis β ∽ exp(c/√n), where n is the dimension of the basis, and the error itself decreases as ∊ ∽ exp(-d√n). Different optimum basis parameters are obtained depending on which quantity one wants to minimize, e.g., the error of the energy expectation value, the distance in Hilbert space, the variance of the energy, or the density at the nucleus. © 1994 John Wiley & Sons, Inc.
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  • 48
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    International Journal of Quantum Chemistry 51 (1994), S. 473-485 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quantum Chemistry can today boast the fact that ordinary chemists - by means of personal computers and programs available - can study many theoretical properties of molecules by solving the Schrödinger equation and get an advance idea of how to properly arrange their experiments to find new features. For small molecules, they can use ab initio programs of the Hartree-Fock type, and for large molecules, they can use semiempirical programs available. To achieve higher accuracy and to include electron correlation properly, however, one has in the ab initio approach to use configurational interaction methods and giant computers, whereas in the semiempirical methods, the effect of correlating is often taken into account in the adjustable parameters forming the basis for this approach. In connection with the ab initio methods, it is further emphasized that the resolvent methods combined with the partitioning technique provide an excellent conceptual and mathematical framework for getting solutions of any accuracy desired, but that most of the programming of this approach remains to be done. Ordinary wave mechanics is valid at absolute zero of temperature, and - in order to include such important chemical concepts as temperature, entropy, free energy, etc. - one has to go over to general quantum theory and the Liouvillian formalism. One can stil start from the Coulombic Hamiltonian, but it becomes of importance to include the nuclear motion properly and preferably on the same level as that of the electronic motion. It is further emphasized that the irreversibility problem is not yet fully solved, that the theory of the interaction between matter and electromagnetic fields still needs some improvements, and that the question of the proper introduction of relativistic corrections in the quantum theoretical treatment of molecular systems involving heavier atoms still has a great deal to desire. © 1994 John Wiley & Sons, Inc.
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  • 49
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    International Journal of Quantum Chemistry 51 (1994), S. 499-518 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electronic rearrangement is a common occurrence in interactions of an atom with other atoms or with solid surfaces and in molecular photodissociation by visible and UV light. In this contribution, the emphasis is on the way properties change over time, as a system progresses from its initial to its final state in a collision. An overview is presented of our recent work describing electronic rearrangement in terms of coupled differential equations for electronic orbitals and nuclear degrees of freedom. An eikonal/(time-dependent Hartree-Fock) approximation is described and a new compact formulation is given in terms of operators in Liouville space, for both small and extended molecular systems. A method is described for treating the coupling of degrees of freedom with very different time scales. The method is illustrated with selected results for temporal atomic population changes in ion-atom and ion-solid surface interactions. © 1994 John Wiley & Sons, Inc.
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  • 50
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    International Journal of Quantum Chemistry 51 (1994), S. 539-553 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The availability of recent supercomputers and massively parallel computing facilities makes possible the calculation of the electronic structure of highly extended (mesoscopic) molecular networks. Disorder, which is practically always present in these systems, causes an extreme complexity of the wave function that typically shows multifractal behavior in the intermediate length scale. Multifractal analysis, however, is possible only on systems that cover several orders of length scales. Though such calculation can be carried out on model systems, it is beyond the bounds of present ab initio or semiempirical treatments. In this contribution, a shape-analysis method of the wave function is given that is applicable both for localized and multifractal one-particle states even in moderately large networks without a regular geometrical structure. No boxing of the distributions is necessary through several orders of magnitude of scaling distances. Multifractal behavior and different regularly decaying localization shape functions can be distinguished. Finite-size multifractal distributions are also discussed. The described method is intended to serve as an easily applicable and efficient tool for bridging over the gap between the wave-function analysis of systems containing macroscopic and moderately large number of particles. © 1994 John Wiley & Sons, Inc.
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  • 51
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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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  • 52
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    International Journal of Quantum Chemistry 52 (1994), S. 9-15 
    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 vibrational Stark effect together with nuclear relaxation and vibrational contributions to the static molecular electric properties of the N2O molecule are computed using ab initio molecular orbital thoery. Contributions to the molecular properties are computed by finite-difference techniques involving the energy vs. the uniform electric-field strength. © 1994 John Wiley & Sons, Inc.
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  • 53
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    International Journal of Quantum Chemistry 52 (1994), S. 17-37 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Some alternative methodologies for the construction of reliable charge density models, involving distributed multipole schemes, potential fitted atomic charges, and a recent approach proposed by Ferenczy, are compared and analyzed critically. Ferenczy's method, which combines the distributed multipole description and the classical least-squares fit, allows one to get an insight into the well-known shortcomings of standard potential- or field-derived atomic charge models. It is now demonstrated that the conformational dependence of fitted point charges is not exclusively due to through-space inductive effects, but that it can be partly rationalized by the reorientation of the atomic multipoles. © 1994 John Wiley & Sons, Inc.
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  • 54
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    International Journal of Quantum Chemistry 52 (1994), S. 49-57 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The generator coordinate approximation (GCA) for the calculation of spectra of diatomics is briefly reviewed. Results of this method applied to a series of curve crossings are presented. In all cases, the generator coordinate method performs significantly better than does the adiabatic approximation. When the energy gap becomes smaller, the GCA results improve. In the crossing limit, the GCA becomes exact. © 1994 John Wiley & Sons, Inc.
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  • 55
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    International Journal of Quantum Chemistry 52 (1994), S. 71-88 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The photodissociation dynamics of some organometallic molecules in the lowest repulsive electronically states are reported for the following concurrent primary reactions: (i) the homolysis of a metal-hydrogen bond vs. the heterolytic loss of a carbonyl ligand in HCo(CO)4; (ii) the photoinduced elimination of molecular hydrogen vs. the loss of a carbonyl ligand in H2Fe(CO)4; and (iii) the photoinduced elimination of molecular hydrogen vs. the loss of a mesithylene ligand in H2Os(CO)Mes (Mes = C6H3(CH3))3. The dynamics are simulated quantum mechanically using a time-dependent wavepacket propagation technique on potential energy surfaces obtained from CASSCF/CCI calculations for HCo(CO)4 and H2Fe(CO)4 and from SCF-INO/MRCI calculations for H2Os(CO)Mes. This approach gives a rather detailed view of some important elementary processes that contribute to the photochemistry of these complexes. The nature of the photoactive excited states is determined without ambiguity, as well as the time scales, the branching ratio of the different primary dissociation pathways, and some features of the absorption spectra. © 1994 John Wiley & Sons, Inc.
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  • 56
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    International Journal of Quantum Chemistry 52 (1994), S. 97-108 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio unrestricted Hartree-Fock (UHF), unrestricted second-order Møller-Plesset (UMP2) perturbation, unrestricted coupled cluster (UCCD), and unrestricted quadratic configuration interaction (UQCISD) calculations have been performed on the organic radicals CH3, CH3CH2, CH2CHCH2, CH3CHCOO-, HCOCHCOH, CH3COCHCOH, CH3COCHCOCH3, and CH3COC(CH3)COCH3, using double-zeta and split-valence-plus-polarization basis sets. These radicals are derived from common organic ligands and have been observed in recent experimental work on tris(β-ketoenolato)cobalt(III) complexes. Their geometry has been optimized at the UHF level using the two mentioned basis sets. From these calcuations, values for the isotropic hyperfine coupling constants at the hydrogen atoms are predicted and compared with the experimental results. The usefulness of semiempirical extrapolations based on limited basis sets and treatment of electron correlation effects is carefully analyzed in the examples considered. © 1994 John Wiley & Sons, Inc.
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  • 57
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    International Journal of Quantum Chemistry 52 (1994), S. 947-956 
    ISSN: 0020-7608
    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 studied symmetry breaking in three open-shell systems: CF4+ (D2d and C2v) and CF32+ (D3h) molecular ions. These different Hartree-Fock solutions are employed as starting points to calculate the correlation energy of these ions with perturbative, configuration interaction, and density functional methods. When symmetry-broken or symmetry-adapted wave functions are used, the correlation energy obtained with each method changes the order of stability of CF4+ for a determined symmetry. Density functional methods produce higher correlation energies although they do not alter the order of stability of Hartree-Fock calculations. The behavior of correlation energy with different methods and the characteristics of the symmetry of wave functions are compared. A study of appearance energies for three different channels of the decomposition reaction of ionized carbon tetrafluoride are considered by using different methods with symmetry-broken or symmetry-adapted wave functions to calculate correlation energies. © 1994 John Wiley & Sons, Inc.
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  • 58
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    International Journal of Quantum Chemistry 52 (1994), S. 935-945 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Second-order density functional methods are used to introduce the electron correlation in Hartree-Fock (HF) ab initio electronic energy calculations of three-dimensional potential energy surfaces (PES). We analyze the behavior of these methods in PES calculations by applying them to the Li + FH reaction, which has been considered a prototype of the elementary atom-diatom reactions. This system has been studied also by the usual techniques, allowing a point-by-point (for a total of 317 grid points) comparison for the lowest 2A' adiabatic state. In particular, we compare the results obtained using the HF, Møller-Plesset (MP3 level), and configuration interaction (CISD and MRDCI levels) methods with the corresponding results obtained using the Colle-Salvetti (CS) and Moscardó-San Fabián (MSF) procedures using the HF results as the starting point. We found that the CS and MSF procedures support the prediction of a shallow well in the entrance channel that deepens slightly away from collinearity and disappears for a bond angle Θ 〈 74°. We also found that the constrained saddle-point positions remain essentially constant from Θ = 180°-90° and are clearly in the exit channel as for the MRDCI approach (corresponding to the best results). In conclusion, there is a good overall agreement, but there is a question in which this agreement is less pronounced: the heights of the saddle points including the transition state. In particular, the transition-state height is about 3 kcal/mol higher than the more accurate value obtained with the MRDCI approach. However, the second-order density functional methods have been capable of reducing the HF barrier from 18 to 9 kcal/mol (all of these values obtained by spline interpolation), the latter value being very similar to the CISD result. © 1994 John Wiley & Sons, Inc.
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  • 59
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    International Journal of Quantum Chemistry 52 (1994), S. 1247-1247 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 60
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    International Journal of Quantum Chemistry 52 (1994), S. 1267-1271 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An extended Rayleigh-Ritz method for computing two-sided eigenvalue bounds of the one-dimensional Schrödinger equation without using a complementary method is presented. The method is based on the B-spline approximation over the truncated domain, which results in a generalized banded matrix eigenvalue problem. Numerical results for two multiminima potentials of different natures are presented. © 1994 John Wiley & Sons, Inc.
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  • 61
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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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  • 62
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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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  • 63
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    International Journal of Quantum Chemistry 52 (1994), S. 215-226 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Topological properties of the charge density \documentclass{article}\pagestyle{empty}\begin{document}$ \rho (\vec r) $\end{document} of a series of diatomic molecules, as well as ethane, ethene, and acetylene are calculated at the Hartree-Fock level employing various basis sets, and by the AM1 method. The effect of the core orbitals on the bonding regions in these molecules is examined. The results help to evaluate the utility of AM1 wavefunctions for analyzing the topological properties of the charge density. © 1994 John Wiley & Sons, Inc.
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  • 64
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    International Journal of Quantum Chemistry 52 (1994), S. 339-348 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The hydration energies of the proton, hydroxyl ion, and several inorganic ions were calculated using the multicavity self-consistent reaction field (MCa SCRF) method developed for the quantum-mechanical modeling of rotationally or flexible systems in dielectric media. The ionic complexes H3O+(H2O)4, OH-(H2O)4, NH4+(H2O)4, and Hal-(H2O)4, where Hal = F, Cl, or Br, have been studied. Each complex was divided between five spheres, corresponding to the central ion and four water molecules in their first coordination sphere, respectively. Each cavity was surrounded by a polarizable medium with the dielectric permittivity of water at room temperature (80). The ionic hydration energies of ions were divided into specific and nonspecific parts. After accounting for the cavity-formation energy using scaled particle theory, good agreement between the total calculated and experimental hydration energies was obtained for all ions studied. © 1994 John Wiley & Sons, Inc.
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  • 65
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    International Journal of Quantum Chemistry 52 (1994), S. 395-410 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The field of nonlinear optics in molecules and polymers offers challenging opportunities for theorists on both molecular modeling of new structures with enhanced microscopic response and manifestations of bulk effect in nonlinearity. This article presents an experimental point of view in assessing the current status of various theoretical approaches. Experimental results are presented for two model compounds together with predictions by various theoretical models to discuss issues and future opportunities. Three different types of nonlinear optical processes are discussed: (i) second order, (ii) third order, and (iii) photorefractivity. © 1994 John Wiley & Sons, Inc.
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  • 66
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    International Journal of Quantum Chemistry 52 (1994), S. 451-467 
    ISSN: 0020-7608
    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 paper we investigate the electronic structure and the static longitudinal polarizability of regular finite and infinite chains of polythiophene at the ab Initio Hartree-Fock level. The effects of the inclusion of polarization functions in the basis set and the influence of introducing a torsional twist between adjacent rings are considered. © 1994 John Wiley & Sons, Inc.
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  • 67
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    International Journal of Quantum Chemistry 52 (1994), S. 433-450 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A methodology to characterize large-scale shape of macromolecular conformations is generalized and applied to a simple model of branched polymers. The standard global analysis of polymer configurations uses geometric shape descriptors, such as the mean radius of gyration. However, geometric descriptors alone do not depict well the intrincate folding features found in many macromolecules. For a more complete characterization, we employ here a new class of shape descriptors which convey the degree and complexity of the “entanglements” in a branched chain. Recently, the methodology has been applied to study statistical properties of linear polymer chains. In the present work, we extend the technique to branched polymers and analyze the configurational and long-chain behavior of their entanglement descriptors. A family of “stars” (polymers with a single branchpoint) with excluded volume interaction is studied, and the results are contrasted with those for linear chains. The relation between changes in geometric and shape descriptors within the polymer conformational space is presented. The complexity of entanglements for large chains and large stars are compared. The results suggest a similar scaling behavior of the entanglement descriptors for the two polymer architectures. © 1994 John Wiley & Sons, Inc.
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  • 68
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    International Journal of Quantum Chemistry 49 (1994), S. 25-34 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The radial Schrödinger equation for the Kratzer molecular potential is equivalent to that of a radial Coulomb problem with an effective (nonintegral) value of rotational angular momentum. The radial Coulomb and the Morse oscillator problems provide different realizations of the algebra so(2,1), whereby the Casimir operators of the Coulomb and Morse oscillator problems are related to the angular momentum quantum number and to the energy, respectively. These relationships permit mappings between the Kratzer molecular potential and the Morse oscillator potential such that the vibrational energy levels of a Kratzer potential with a fixed rotational angular momentum quantum number may be mapped onto degenerate vibrational levels of a set of displaced Morse oscillators. The ground vibrational level of the Kratzer potential is mapped onto the ground vibrational level of a specific Morse oscillator and the remaining (infinite) set of higher vibrational levels are mapped onto degenerate states of displaced Morse oscillators, corresponding to systematic unit increase in the number of bound vibrational levels and successive decrease in equilibrium separation. This behavior is contrasted with that of the finite set of displaced Morse potentials arising as supersymmetric partner potentials to a given parent Morse potential, where there is a systematic unit decrease in the number of bound vibrational levels and a successive increase in equilibrium separation. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 49 (1994), S. 63-66 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 70
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    International Journal of Quantum Chemistry 49 (1994), S. 135-144 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 71
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    International Journal of Quantum Chemistry 49 (1994), S. 159-169 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The definition of local hardness is clarified within the Kohn-Sham formulation of the density functional theory. Exact equations for the local hardness of open- and closed-shell electronic systems are derived. It is found that the Kohn-Sham independent particle kinetic energy does not contribute to local hardness. The apparent contradiction with previous work of Parr et al. is cleared up when the local hardness definition is revised. © 1994 John Wiley & Sons, Inc.
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  • 72
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    International Journal of Quantum Chemistry 49 (1994), S. 207-213 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Parr and Chattaraj proposed a principle of maximum hardness for stable molecular structures. Pearson and Palke used ab initio SCF MO calculations for ammonia and ethane to demonstrate the operation of the principle. In this paper, we present ab initio SCF MO results for five isomeric forms of the homoatomic P6 cluster as further support for the principle of maximum hardness. © 1994 John Wiley & Sons, Inc.
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  • 73
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    International Journal of Quantum Chemistry 49 (1994), S. 197-206 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The previously introduced σ and π energy separation is extended to subsystems. σ and π electron donor and acceptor properties of the substituents are tested with a Löwdin population analysis. The influence of the substituents on the σ and π electron energies is calculated for substituted benzene, cyclobutadiene, butadiene, and borazine. A linear relationship is found which shows that an increase of σ electron energy is accompanied by a decrease in π electron energy and vice versa. This allows a classification of substituent groups in terms of reactivity. © 1994 John Wiley & Sons, Inc.
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  • 74
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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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  • 75
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    International Journal of Quantum Chemistry 52 (1994), S. 595-610 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electronic structure investigations on a broad range of gold compounds, including naked and ligated gold clusters, are reviewed. The calculations have been carried out with a recently introduced relativistic variant of the linear combination of Gaussian-type orbitals density-functional (LCGTO-DF) method which affords all-electron investigations for very large systems. The accuracy of the method will be evaluated for the gold dimer. Then the electronic structure of the naked cluster Au55 is studied, both in Ih and Oh symmetry. Nonrelativistic and relativistic results obtained by the present method are compared to those of the much simpler jellium model. Since triphenylphosphine is among the most common ligands in gold chemistry a series of mononuclear gold phosphine compounds MeAuPR3 with increasingly complex ligands PR3 (R = H, CH3, C5H6) is discussed. The calculations reveal the success and the limitations of simpler phosphines often employed as model ligands in theoretical studies. Some aspects of the phosphine gold interaction in these simpler compounds carry over to the main group element centered gold clusters. Thereby one arrives at a rationalization of the particularly high stability of the carbon-centered octahedral cluster cation [(R3PAu)6C]2+ as compared to the neighboring isoelectronic boron and nitrogen-centered clusters. © 1994 John Wiley & Sons, Inc.
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  • 76
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    International Journal of Quantum Chemistry 52 (1994), S. 633-644 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Density-Functional method, with Linear Combination of Atomic Orbitals, has been applied to eight crystals: the lattice equilibrium parameters, and the lattice formation energies have been calculated at the Hartree-Fock level (HF), at the hybrid Hartree-Fock Density-Functional level (DFT/HF), and at the Kohn-Sham Density-Functional level (DFT). The band structures and the electronic charge distributions calculated at the DFT and HF levels are compared. © 1994 John Wiley & Sons, Inc.
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  • 77
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    International Journal of Quantum Chemistry 52 (1994), S. 645-654 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interaction of the CO2 molecule with a single palladium atom has been studied by means of a linear combination of Gaussian type orbitals-density functional (LCGTO-DF) method, as implemented in the code deMon. Local and nonlocal functionals were used. The core electrons of the palladium atom were represented by a model core potential, which includes relativistic effects. Several coordination modes of the CO2 moiety to the Pd atom were studied. They were fully optimized by allowing both electronic and structural relaxation. The calculations indicate a dihapto CO (η2-CO) coordination mode (or mixed Pd—C/Pd—O bonding), of Cs symmetry, as the one of lowest energy. There is a strong charge transfer from Pd to CO2, which produces a large structural change in CO2: It goes from a linear to a bent geometry. The calculated frequencies of CO2 in Pd—CO2 are 689, 1212, and 2018 cm-1 for the bending, symmetric, and asymmetric vibrational modes, respectively. These results agree reasonably with the observed frequencies of CO2 adsorbed on H2O/Pd(110) and Pd(111)/Na surfaces. The η2-CO structure is a likely candidate for the adsorption of CO2 on palladium surfaces, since the asymmetric mode is observed. This mode is dipole allowed in Cs symmetry but dipole forbidden in C2v. The monohapto C (η1-C) coordination mode, of C2v symmetry, was found to be a transition state, 2.2 and 8.4 kcal/mol, for the local and nonlocal levels, respectively, above the (η2-CO) minimum. The η2-O—O mode is not attainable for Pd—CO2; calculations started with this configuration converge to the η2-CO mode, the ground state. This is a quite different picture to that found for Ni—CO2, where the moiety is chemisorbed in the dihapto η2-O—O coordination mode. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 52 (1994), S. 695-710 
    ISSN: 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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  • 79
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: High-Tc superconductors (HTSCS) are usually obtained by doping electron donors or acceptors into parent materials. The actual role played by doping is still uncertain with various interpretations. The present electronic structure study provides some hints which may help to solve the mystery. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 52 (1994), S. 527-548 
    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 theoretical investigation on the core and valence photoelectron spectra of unsaturated hydrocarbons of increasing complexity, ethylene, trans-butadiene, and trans-hexatriene, has been performed in order to analyze the role of the most important many-body effects on the spectral features and the evolution of these effects along the series. The capability of various configuration interaction (CI) schemes to interpret both the core and the valence ionization satellite structures has been pointed out. The 3h-2v(π)-1p and 2h-1p CI schemes employed, together with an accurate choice of the basis set and the explicit inclusion of the relaxation, has proven to be very useful tools for this kind of study, being sufficiently accurate to explain the most important correlation features and extendible, at the same time, to longer chain systems. © 1994 John Wiley & Sons, Inc.
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  • 81
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    International Journal of Quantum Chemistry 52 (1994), S. 575-591 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The authors proposed a general method to analyze chemical phenomena in such static states as resonance and aromaticity, without changing the variational space of the trial wave function. Solution of the Hartree-Fock-Roothaan (HFR) equation for a molecular system gives the ground-state electronic structure and its energy. One can impose constraint conditions, which have chemical meanings, on the HFR equation (the constrained HF equation). The presented method is to analyze the energy differences between usual and constrained HF solutions. This method has an advantage over conventional methods in that it neither varies the trial wave function nor the geometry. The method was applied to interpretation of resonance energy in unsaturated linear hydrocarbons to show that both vertical and adiabatic resonance energies are produced proportionally to the number of ethylene units and that they originate by the lowering of the kinetic energy of π electrons, namely, resonance releases the kinetic-energy pressure of π electrons. This is consistent with the previous investigations © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 49 (1994), S. 613-623 
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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: Self-consistent Kohn-Sham density functional calculations have been carried out to study the structure of the ammonia dimer. The local-density approximation yields unusually large binding energy and short internitrogen distance compared with the experimental and more accurate theoretical data. The results from the Becke-Perdew gradient-corrected functionals are generally in good agreement with those at the SCF MP2 level when the geometry is fully optimized with various large basis sets. With our best estimation, the staggered quasi-linear structure (Cs) is 0.6 kcal/mol lower in energy than the symmetric cyclic one (C2h). The hydrogen-bonded N - H bond in the staggered quasi-linear structure is found to be 0.008 Å longer than the N - H bond in ammonia. In our calculations, we could not find the minima on the energy surface corresponding to the two asymmetric cyclic structures suggested by microwave spectra and coupled pair functional calculations. © 1994 John Wiley & Sons, Inc.
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    International Journal of Quantum Chemistry 52 (1994) 
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    International Journal of Quantum Chemistry 52 (1994), S. 695-704 
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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: Three representative isomerization reactions (HNC → HCN, CH3NC → CH3CN, and N2H2 trans → cis and sin) have been studied using both the LCGTO-LSD and LCGTO-NLSD density functional methods and employing a new algorithm for the search and the refinement of the transition-state structures. The inclusion of the nonlocal corrections and the use of large basis sets improve the reliability of the energetic parameters. Results are in good agreement with previous accurate first-principle computations and available experimental data. © John Wiley & Sons, Inc.
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  • 85
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    International Journal of Quantum Chemistry 52 (1994), S. 731-765 
    ISSN: 0020-7608
    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 implementation of the intrinsic reaction coordinate (IRC) method within the density functional theory (DFT) framework is presented. The implementation has been applied to four different types of chemical reactions represented by the isomerization process, HCN = HNC (A); the SN2 process, H- + CH4 = CH4 + H- (B); the exchange process, H· + HX = HX + H· (X = F,Cl) (C); and the elimination process, C2H5Cl = C2H4 + HCl (D). The present study presents for each process optimized structures and calculated harmonic vibrational frequencies for the reactant(s), the transition state, and the product(s) along with the IRC path connecting the stationary points. The calculations were carried out within the local density approximation (LDA) as well as the LDA/NL scheme where the LDA energy expression is augmented by Perdew's and Becke's nonlocal (NL) corrections. The LDA and LDA/NL results are compared with each other as well as the best available ab initio calculations and experimental data. For reaction (D), ab initio calculations based on MP2 geometries and MP4SDTQ energies have been added due to the lack of accurate published post-HF calculations on this process. A detailed discussion is provided on the efficiency of the IRC algorithms, the relative accuracy of the DFT and ab initio schemes, as well as the reaction mechanisms of the four reactions. It is concluded that the LDA/NL scheme affords the same accuracy as does the MP4 method. The post-HF methods seem to overestimate activation energies, whereas the corresponding LDA/NL estimates are too low. The LDA activation energies are even lower than the LDA/NL counterparts. The incorporation of the IRC method into the DFT framework provides a promising and reliable tool for probing the chemical reaction path on the potential energy surfaces, even for large-size systems. IRC calculations by ab initio methods of an accuracy similar to the LDA/NL scheme, such as the MP4 scheme, are not feasible. © John Wiley & Sons, Inc.
    Additional Material: 15 Ill.
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  • 86
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    International Journal of Quantum Chemistry 52 (1994), S. 809-816 
    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 first discussion of the dynamics of Jahn-Teller systems in terms of the electronic density as the fundamental variable was given by W.J. Clinton in 1960, where the degenerate electronic configuration of a Jahn-Teller molecule was interpreted in terms of the infinite number of ways in which the charge distribution can be oriented for the same energy. The moving nuclear framework serves as the perturbation necessary to define the orientation of the charge density, with no activation energy required to put the charge cloud into motion. Recently, this notion of the electronic charge cloud in a Jahn-Teller molecule sweeping out the potential surface over which the nuclei move has found mathematical expression in our work in terms of a generalized electronic current density in nuclear-coordinate space [N. Sukumar and B.M. Deb, Int. J. Quantum Chem. 40, 501 (1991)]. The introduction of the electronic phase as a function of both electronic and nuclear coordinates, in addition to the electronic density, is a crucial component of this formulation. In the present work, the density-based treatment is extended to the nonadiabatic situation, with the Born couplings interpreted as nonadiabatic currents in parameter space. Abelian and non-Abelian gauge transformations of these currents are discussed. © John Wiley & Sons, Inc.
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  • 87
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    International Journal of Quantum Chemistry 52 (1994), S. 837-847 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Density functional theory (DFT) using gradient-corrected «nonlocal» functionals is used to calculated the thermochemistry and barrier heights for several types of peroxyl radical isomerizations currently being studied by kineticists. The calculations are generally in good agreement with experimental data, where such data are available. An important exception is that the O - H bond strengths in hydroperoxides are all predicted to be too weak by about 7 kcal/mol. The calculated reaction barriers are a few kcal/mol lower than the experimental estimates, but comparable in accuracy to the much more computationally expensive second-order Møller-Plesset (MP2) predictions. Various theoretical methods converge on a 42 ± 2 kcal/mole barrier for CH3OO → H2CO + OH. The DFT calculations can be used to predict reaction barriers in cases where no reliable experimental data are available. The effects of the choice of basis set and correlation functional are explored. Improvements needed to make these calculations most valuable to the chemical kineticist are discussed. © 1994 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
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  • 88
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spin-polarized Xα-SW calculations of [Fe6(μ3-S)8(PH3)6]2+ as a model of the cluster [Fe6(μ3-S)8(PEt3)6] (BPh4)2 have been performed. The highest occupied energy levels are well separated from empty levels, and up to a maximum of eight electrons can be unpaired, giving a maximum spin state with S = 4. This electronic state is consistent with the magnetic data of [Fe6(μ3-S)8(PEt3)6](BPh 4)2, which have been interpreted using the Heisenberg-Dirac-Van Vleck exchange spin Hamiltonian. The S = 4 state arises from the magnetic coupling between five low-spin (Si = 1/2) and one intermediate-spin (S = 3/2) iron(III) center. © 1994 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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  • 89
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    International Journal of Quantum Chemistry 52 (1994), S. 927-934 
    ISSN: 0020-7608
    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 cryolite is investigated with a theoretical approach based on LDF calculations. In fact, experimental techniques for structural studies are difficult to perform in cryolite melts because of hard experimental conditions (high temperature, corrosiveness, etc.). Use of the DMol software allows us to determine the stabilities and the vibrational frequencies of AlF63-, AlF52-, and AlF4- isolated complexes. The results obtained compared with published experimental works confirm that AlF52- should be considered in the dissociation scheme of cryolite as previously evidenced by other authors. © 1994 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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  • 90
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    International Journal of Quantum Chemistry 52 (1994), S. 973-985 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bonding of acetylene to copper atom, dimer, and trimer was investigated with a Kohn-Sham density functional approach. Full geometry optimization yielded the equilibrium structures of various CunC2H2 species. Gradient corrections were included in the calculation of binding energies (BE). The Cu - C2H2 complex was found to have a Cs structure and a BE of 10 kcal/mol. Three isomers of Cu2C2H2 have similar total energies: a C2v end-bonded structure with a BE of 18 kcal/mol, and two 1,2-dicupro ethylene isomers - a cis form with a BE of 12 kcal/mol and a trans form with a BE of 15 kcal/mol. Two stable C2v isomers of Cu3C2H2 were found. In both isomers, the Cu3 ring relaxes from its isosceles structure, with two short bonds (2.247 Å) and one long bond (2.478 Å), and adopts a nearly equilateral geometry. In one isomer of Cu3C2H2, the acetylene is bonded to one apex of the Cu3 ring with a BE of 29 kcal/mol. In the other, it is bonded to two copper atoms of one side of the Cu3 ring with a BE of 33 kcal/mol. © 1994 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 91
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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.
    Additional Material: 5 Ill.
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  • 92
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    New York, NY : Wiley-Blackwell
    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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  • 93
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    International Journal of Quantum Chemistry 52 (1994), S. 1113-1119 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optical properties and stability of hydrogenated silicon clusters are investigated using density functional pseudopotential calculation. The dipole transitions between the band-edge orbitals are allowed, in contrast to the indirect gap in bulk silicon. Evolution of a small amount of hydrogen atoms enhances the dipole transition, increasing the photoluminescence intensity. Further dehydrogenation creates gap states due to dangling bonds, which may decrease the photoluminescence intensity via nonradiative recombination processes. The Stokes shift is also estimated by calculating the relaxed structure of the excited state within the local density approximation. © 1994 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
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  • 94
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    International Journal of Quantum Chemistry 52 (1994), S. 1153-1176 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The recently introduced set of the quadratic, two-electron covalent and ionic valence indices is used to investigate the bond-breaking-bond-forming (BB-BF) process in an atom exchange reaction between H2 and X (X = H, F - I) as well as in the O2 - H system. Valence changes accompanying selected charge reorganizations are examined within the three-orbital model and valence diagrams for symmetric transition states (TSs) are given. The UHF valence data for Li2O and CO2 and the H - H - X, O - O - H, and O - H - O (ABC) TSs (collinear and angular) are reported and compared to valence data in the separated fragments limits (SFL), AB and BC. The overall valence, ν(ABC), and the total (ionic plus covalent) diatomic valences, νAB and νBC, are used as measures of the overall bond-order in a concerted BB-BF reaction, to test the postulate of the bond-energy-bond-order (BEBO) model. In collinear TSs of H2X, ν ≊ -1, i.e., one bonding electron pari, is found to be roughly preserved, whereas in the angular H2X and in collinear O - H - O TSs, the effect of increased valence at the saddle-point is observed, relative to that of diatomic fragments (reactiants or products). For the angular O - O - H TS, a similar increase in | ν (ABC)| relative to both O2 and OH SFLs is detected; smaller changes relative to the O2 data are found in the collinear TS. This observation is in agreement with earlier predictions from the intersecting-state model. The relative diatomic valences, νABTS/νABSFL and νBCTS/νBCSFL, are shown to conserve the overall relative bond multiplicity around 1 in both collinear and angular TSs of the H2 - X systems. © 1994 John Wiley & Sons, Inc.
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  • 95
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    International Journal of Quantum Chemistry 52 (1994), S. 349-360 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The PM3 quantum-mechanical method has been used to study large water clusters ranging from 8 to 42 water molecules. These large clusters are built from smaller building blocks. The building blocks include cyclic tetramers, pentamers, octamers, and a pentagonal dodecahedron cage. The correlations between the strain energy resulting from bending of the hydrogen bonds formed by different cluster motifs and the number of waters involved in the cluster are discussed. The PM3 results are compared with TIP4P potential and ab initio results. The number of net hydrogen bonds per water increases with the cluster size. This places a limit on the size of clusters that would fit the Benson model of liquid water. Many of the 20-mer clusters fit the Benson model well. Calculations of the ion cluster (H2O)40(H3O+)2 reveal that the m/e ratio obtainable by mass spectrometry experiments can uniquely indicate the conformation of the 20 water pentagonal dodecahedron cage present in the larger clusters. © 1994 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
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  • 96
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    International Journal of Quantum Chemistry 49 (1994), S. 105-127 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction path is shown to be always a gradient line on a potential energy surface (PES) of a molecule. The properties of gradient lines on the PES are elucidated. Correct symmetry conservation rules along the gradient line are derived. The behavior of the gradient line on a PES with different topologies are considered. © 1994 John Wiley & Sons, Inc.
    Additional Material: 11 Ill.
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  • 97
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    International Journal of Quantum Chemistry 49 (1994), S. 129-130 
    ISSN: 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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  • 98
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    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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  • 99
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    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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  • 100
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    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
    Type of Medium: Electronic Resource
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