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  • 1
    Electronic Resource
    Electronic Resource
    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
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459831
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  • 2
    Electronic Resource
    Electronic Resource
    Second-order Møller–Plesset perturbation theory as a configuration and orbital generator in multiconfiguration self-consistent field calculations (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3834-3839 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown that the magnitude of the natural orbital (NO) occupation numbers of second-order Møller–Plesset (MP2) perturbation theory can be used to select physically reasonable configuration spaces for ground state MCSCF calculations. When the MP2 NO's are used as an initial guess for the orbitals, a second-order Newton–Raphson MCSCF calculation is in the local region from the first iteration. Fast convergence is therefore ensured to a stationary point with orbitals of similar structure as the MP2 NO's, thereby reducing significantly the risk of converging to undesired stationary points.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453884
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  • 3
    Electronic Resource
    Electronic Resource
    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
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459448
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  • 4
    Electronic Resource
    Electronic Resource
    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
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455749
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  • 5
    Electronic Resource
    Electronic Resource
    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
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455063
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  • 6
    Electronic Resource
    Electronic Resource
    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
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457471
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  • 7
    Electronic Resource
    Electronic Resource
    Linear response calculations for large scale multiconfiguration self-consistent field wave functions (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 3654-3661 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown that large scale MCSCF linear response (MCLR) calculations can be carried out efficiently using an iterative algorithm where the linear transformations are carried out directly, i.e., without explicitly constructing the MCLR matrices. Calculations are presented on H2O of frequency dependent polarizabilities with configuration spaces containing up to 128 283 determinants.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454885
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  • 8
    Electronic Resource
    Electronic Resource
    Molecular equilibrium structures from experimental rotational constants and calculated vibration–rotation interaction constants (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 6482-6496 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A detailed study is carried out of the accuracy of molecular equilibrium geometries obtained from least-squares fits involving experimental rotational constants B0 and sums of ab initio vibration–rotation interaction constants αrB. The vibration–rotation interaction constants have been calculated for 18 single-configuration dominated molecules containing hydrogen and first-row atoms at various standard levels of ab initio theory. Comparisons with the experimental data and tests for the internal consistency of the calculations show that the equilibrium structures generated using Hartree–Fock vibration–rotation interaction constants have an accuracy similar to that obtained by a direct minimization of the CCSD(T) energy. The most accurate vibration–rotation interaction constants are those calculated at the CCSD(T)/cc-pVQZ level. The equilibrium bond distances determined from these interaction constants have relative errors of 0.02%–0.06%, surpassing the accuracy obtainable either by purely experimental techniques (except for the smallest systems such as diatomics) or by ab initio methods. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1459782
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  • 9
    Electronic Resource
    Electronic Resource
    An analysis and implementation of a general coupled cluster approach to excitation energies with application to the B2 molecule (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 671-679 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general scheme is presented for the calculation of excitation energies using the standard coupled cluster hierarchy and a simple implementation is described for the higher standard models. An error analysis is performed to find to what order excitation energies in different coupled cluster models are correct. The analysis includes both the standard coupled cluster hierarchy as well as the approximate models and considers excitations to states that are dominated by one, two, and three electron replacements compared to the reference state. Calculations are presented up to the quadruple excitation level for the open shell B2 molecule using an excited closed shell state as reference state to emphasize the usefulness of the order analysis. The coupled cluster excitation energies are compared to full configuration interaction results. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1379332
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  • 10
    Electronic Resource
    Electronic Resource
    The initial implementation and applications of a general active space coupled cluster method (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7140-7148 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general coupled cluster method that allows arbitrary excitations from a single reference-determinant is proposed and tested. The method is based on a generalization of the formalism of spin-strings and provides a unified method for the storage and manipulation of coupled cluster operators. An initial implementation of the method is discussed and used to study the convergence of the coupled cluster hierarchy for H2O and CH2 at equilibrium geometry, employing up to eightfold excitations. The energy and wave function contributions of the various excitation levels are examined. The dissociation curve of HF is also studied. Using single and double excitations from a minimal active space, the coupled cluster dissociation curve for HF shows a largest deviation from full configuration interaction curve of 1.3 mEh, which decreases by an order of magnitude up on the addition of triple excitations out of the active space. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1290005
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