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  • 1
    Electronic Resource
    Electronic Resource
    A chemical application of the algebraic concept of internal space (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2538-2544 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a general method for extracting the internal space of an arbitrary wave function, exploiting the molecular symmetry. This permits the rewriting of a wave function built with nonorthogonal orbitals as a compact expansion of Slater determinants over orthogonal orbitals. The method is applied to valence bond (VB) and projected unrestricted Hartree–Fock (PUHF) functions along the potential curves of some diatomics, namely He2+, F2−, and F2. These functions are then used as references for further configuration interaction and perturbative Møller–Plesset calculations. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475637
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  • 2
    Electronic Resource
    Electronic Resource
    Variational spaces of electronic calculations in quantum chemistry (1994)
    Springer
    Journal of mathematical chemistry 15 (1994), S. 303-321 
    Details
    ISSN: 1572-8897
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mathematics
    Notes: Abstract The exterior algebra formalism is presented, then used to obtain, in a very simple way, Brillouin's theorem, and to derive the general algebraic equations of the various variational spaces explored by RHF, ROHF, UHF, CASSCF, UCASSCF, CI methods. When a given basis set of one-electron orbitals (not necessarily orthogonal) is fixed, these equations lead to analytical equations for the CI coefficients only. The important algebraic concepts (i.e. concepts that do not refer to any particular basis set) of internal space, length and factorization of a multi-configuration are also introduced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01277567
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  • 3
    Electronic Resource
    Electronic Resource
    A mathematical definition of molecular structure – open problem (1998)
    Springer
    Journal of mathematical chemistry 23 (1998), S. 61-63 
    Details
    ISSN: 1572-8897
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mathematics
    Notes: Abstract A conjecture is proposed whose interest lies in the fact that, if false, it tells us what a molecule is not, if true, it is the basis for a rigorous definition of molecular structure in quantum mechanics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019156821626
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  • 4
    Electronic Resource
    Electronic Resource
    Ensemble-representable densities for atoms and molecules. II. Application to CoCl4 2- (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 667-680 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the previous article we introduced a method to obtain an ensemble density describing a molecule in a crystal from diffraction experiment structure factors. Here the method is applied to the CoCl42- molecular ion in a Cs3CoCl5 crystal for which accurate magnetic structure factors are known. First, the approximations involved in the interpretation of polarized neutron experiment are reviewed with special emphasis on the collinearity approximation which has been avoided in this work. Second, the derivation of magnetic structure factors corresponding to theoretical ensemble densities is explained (the spin and the exact orbital contributions have been included). Third, the fitting procedure is presented and results at different levels of approximation are discussed. The main conclusions are: (1) A density built by using several molecular wave functions can give a very good agreement with the experimental data. (2) The ensemble representability constraint is necessary to retrieve physical information from the optimized parameters. (3) Taking into account the proper paramagnetic contribution to the magnetization improves significantly the agreement between theory and experiment. (4) Neglecting the diamagnetic contribution and the fact that the magnetization may be locally noncollinear to the applied external field is fully justified for the system under study. © 1996 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Spin-unrestricted calculations in quantum chemistry (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 46 (1993), S. 593-607 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The unrestricted complete active space self-consistent field (UCASSCF) function is defined, and a proof that a UCASSCF eigenfunction of the spin operator S2 is a CASSCF function is given. The spin-contamination for an unrestricted Hartree-Fock (UHF) function is evaluated by using Araki angle operators, and the UHF function is then projected on the restricted open-shell Hartree-Fock (ROHF) space. The present analysis has deep consequences since it implies that the only non-spin-contaminated UHF functions are the ROHF functions. This is illustrated in a calculation of the spin density of He2+. © 1993 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560460502
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  • 6
    Electronic Resource
    Electronic Resource
    Ensemble representable densities for atoms and molecules. I. General theory (1995)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 54 (1995), S. 201-210 
    Details
    ISSN: 0020-7608
    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 to obtain ensemble representable densities from experimental diffraction data is proposed. The method uses ab initio molecular densities instead of the commonly employed one-electron orbital densities, and as a result, few parameters need to be optimized in the fitting procedure to the experimental structure factors. The optimized coefficients can provide information about intra- and intermolecular electronic correlations, spin-orbit coupling, etc. This work also provides new explicit formulas to determine the rank of a fermionic wave function, i.e., the rank of the one-fermion density matrix. © 1995 John Wiley & Sons, Inc.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560540403
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  • 7
    Electronic Resource
    Electronic Resource
    Symmetrizing broken symmetry wave functions in quantum chemistry (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 68 (1998), S. 91-101 
    Details
    ISSN: 0020-7608
    Keywords: symmetry breaking ; unrestricted wave function ; Löwdin projector ; group algebra projector ; nonorthogonal configuration interaction ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general scheme to symmetrize a broken symmetry wave function is described. It offers great flexibility in the choice of the set of functions used to expand the symmetrized functions. The traditional symmetrization approaches are related to some particular choices of these functions. New choices are also considered. A postsymmetrization treatment is proposed.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 91-101, 1998
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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