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  • Artikel  (97)
  • American Institute of Physics (AIP)  (60)
  • Cambridge University Press  (37)
  • Chemie und Pharmazie  (60)
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  • Artikel  (97)
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  • 1
    Digitale Medien
    Digitale Medien
    Bifurcation analysis of chemical reaction mechanisms. I. Steady state bifurcation structure (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 3083-3095 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The vocabulary and techniques of numerical bifurcation analysis are described, with an emphasis on steady state bifurcations of codimension one and two. The direct computation of bifurcation sets is shown to be of considerable utility in analyzing and comparing complex chemical reaction mechanisms. The systems chosen for analysis are the chlorite–iodide and the mixed Landolt reactions. The calculation of a simple hysteresis loop for a mechanism of the chlorite–iodide reaction using both numerical bifurcation analysis and numerical integration begins an extended comparison between the methods advocated in this paper and more familiar methods. The systematic identification of the existence of isolated branches of steady states is described for a second mechanism of the chlorite–iodide reaction. Two mechanisms for the mixed Landolt system are contrasted. It is found that the alternative negative feedback pathway mechanism, which reproduces the periodic behavior more successfully at a selected point in parameter space, is less satisfactory than the EOE mechanism in describing the overall bifurcation behavior of the system in the parameter plane for which experimental data are available. Finally, we examine another mechanism for the chlorite–iodide reaction. This mechanism was designed with the intent of suppressing oscillatory behavior on a subset of parameter space. Numerical integration revealed only bistability of steady states, as desired, but numerical bifurcation analysis reveals a sizeable region in the previously investigated subset of parameter space on which oscillatory behavior exists.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.459831
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  • 2
    Digitale Medien
    Digitale Medien
    Quadratic response functions for a multiconfigurational self-consistent field wave function (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 1174-1190 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We describe an efficient implementation of the quadratic response function for a multiconfiguration self-consistent field reference wave function. The quadratic response function determines the hyperpolarizability and its residues determine the two-photon transition matrix elements and the transition matrix elements between excited states. We report sample calculations for the hyperpolarizability of Ne and for the two-photon transition matrix elements of Ne and H2.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.463245
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  • 3
    Digitale Medien
    Digitale Medien
    Finite-element multiconfiguration Hartree–Fock calculations on the excitation energies and the ionization potential of oxygen (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 5229-5232 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The 3P, 1D, and 1S states of oxygen and the 4S ground state of the oxygen cation have been studied using large numerical multiconfiguration Hartree–Fock calculations with systematically increased active spaces. The nonrelativistic valence limits for the excitation energies are 1.968 and 4.181 eV for E(3P–1D) and E(3P–1S), respectively. The nonrelativistic valence limit for the ionization potential is 13.593 eV.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462733
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  • 4
    Digitale Medien
    Digitale Medien
    Large multiconfiguration Hartree–Fock calculations on the hyperfine structure of B(2P) and the nuclear quadrupole moments of 10B and 11B (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 5051-5055 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The hyperfine parameters for B(2P) are studied using numerical multiconfiguration Hartree–Fock (MCHF) calculations. A newly developed finite-element MCHF program allowing very large configuration–interaction expansions is used. The magnetic hyperfine parameters obtained are 0.0879(28), 0.7817(11), and −0.1675(3) a.u. for the Fermi contact term, the orbital term, and the spin–dipolar term, respectively, as compared to the experimental values of 0.1016(62), 0.7783(16), and −0.1686(5) a.u. The nuclear quadrupole moments obtained by comparing computed electric field gradients with experimental nuclear quadrupole couplings constants are Q(10B)=0.084 59(24) b and Q(11B)=0.040 59(10) b.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.460540
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  • 5
    Digitale Medien
    Digitale Medien
    Determinant based configuration interaction algorithms for complete and restricted configuration interaction spaces (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2185-2192 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A restricted active space (RAS) wave function is introduced, which encompasses many commonly used restricted CI expansions. A highly vectorized algorithm is developed for full CI and other RAS calculations. The algorithm is based on Slater determinants expressed as products of alphastrings and betastrings and lends itself to a matrix indexing C(Iα, Iβ ) of the CI vector. The major features are: (1) The intermediate summation over determinants is replaced by two intermediate summations over strings, the number of which is only the square root of the number of determinants. (2) Intermediate summations over strings outside the RAS CI space is avoided and RAS calculations are therefore almost as efficient as full CI calculations with the same number of determinants. (3) An additional simplification is devised for MS =0 states, halving the number of operations. For a case with all single and double replacements out from 415 206 Slater determinants yielding 1 136 838 Slater determinants each CI iteration takes 161 s on an IBM 3090/150(VF).
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.455063
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  • 6
    Digitale Medien
    Digitale Medien
    Spin polarization in restricted electronic structure theory: Multiconfiguration self-consistent-field calculations of hyperfine coupling constants (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 3412-3419 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: It is shown that the isotropic hyperfine coupling constant Aiso associated with the Fermi contact interaction can be accurately calculated from conventional multiconfiguration self-consistent-field wave functions if, in addition to the expectation value of the contact operator, one also includes the terms originating from the response of the wave function. These response terms are nonvanishing only for perturbation operators that are nonsymmetric in spin space. Calculations of Aiso for N and BH2 illustrate the importance of the response terms and also indicate that a good estimate of the spin polarization of the core orbitals may be obtained from the response terms without correlating the core.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462977
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  • 7
    Digitale Medien
    Digitale Medien
    Spin–orbit coupling constants in a multiconfiguration linear response approach (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 2118-2126 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Spin–orbit coupling constants between singlet and triplet states are evaluated as residues of multiconfiguration linear response functions. In this approach, the spin–orbit coupling constants are automatically determined between orthogonal and noninteracting states. Sample calculations are presented for the X 3Σ−g–b 1Σ+g transition in O2 and the 1A1–3B1 transition in CH2. The convergence of the coupling constants is examined as a function of basis set and level of correlation. An exotic behavior is observed in the correlation of the 1A1 state for CH2 when increasing the active space, demonstrating an intricate coupling between the dynamic and static correlation. In general, the results indicate that reliable spin–orbit coupling constants between valence states may be obtained with a 4s3p2d1f basis set for first row atoms and a modest active orbital space.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462063
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  • 8
    Digitale Medien
    Digitale Medien
    C6 dispersion coefficients in multiconfiguration self-consistent field linear response theory (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 7256-7263 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: An algorithm for the evaluation of correlated dipole–dipole dispersion coefficients by direct MCSCF linear response theory is presented. Stepwise construction of a pseudo-state basis using eigenvectors of successive (Cauchy) moments of the linear response function gives an efficient scheme for obtaining polarizabilities at imaginary frequency. The scheme is also useful for polarizabilities on the real axis below the frequency of the first dipole-allowed transition. Sample calculations of C6 coefficients are described for a range of two-electron (H−, He, Li+, Be2+, H2, and H+3 ) and many-electron (Be, N, N2 ) closed- and open-shell systems.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.459448
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  • 9
    Digitale Medien
    Digitale Medien
    Triplet excitation properties in large scale multiconfiguration linear response calculations (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 381-388 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: It is shown that large scale MCSCF linear response (MCLR) calculations can be carried out efficiently when the perturbation operator has triplet symmetry. Methods have been developed to efficiently construct two electron density matrices containing triplet orbital excitation operators and two electron density matrices which connect singlet and triplet states. Direct CI methods have also been developed for two electron operators with spin rank one. Numerical calculations are presented for CH+ and the results compared with full CI results.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.457471
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  • 10
    Digitale Medien
    Digitale Medien
    Erratum: Second-order Møller–Plesset perturbation theory as a configuration and orbital generator in multiconfiguration self-consistent-field calculations [J. Chem. Phys. 88, 3834 (1988)] (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5354-5354 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.455749
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