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  • 1
    Electronic Resource
    Electronic Resource
    Computing coupled-cluster wave functions with arbitrary excitations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 1359-1365 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An algorithm is presented for solving coupled-cluster (CC) equations by successive diagonalization of 2×2 matrices. It is more expensive than usual procedures, but it is capable of solving a CC problem where any arbitrary excitation is included in the cluster operator. Equation-of-motion coupled-cluster (EOMCC) excitation energies can also be determined by this method regardless of the type of excitations in the cluster operator and the space where the effective Hamiltonian is diagonalized. The algorithm is applied to the study of the convergence of CC and EOMCC series in some small bases. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481925
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  • 2
    Electronic Resource
    Electronic Resource
    A general state-selective multireference coupled-cluster algorithm (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 980-990 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A state-selective multireference coupled-cluster algorithm is presented which is capable of describing single, double (or higher) excitations from an arbitrary complete model space. One of the active space determinants is chosen as a formal Fermi-vacuum and single, double (or higher) excitations from the other reference functions are considered as higher excitations from this determinant as it has been previously proposed by Oliphant and Adamowicz [J. Chem. Phys. 94, 1229 (1991)]. Coupled-cluster equations are generated in terms of antisymmetrized diagrams and restrictions are imposed on these diagrams to eliminate those cluster amplitudes which carry undesirable number of inactive indices. The corresponding algebraic expressions are factorized and contractions between cluster amplitudes and intermediates are evaluated by our recent string-based algorithm [J. Chem. Phys. 115, 2945 (2001)]. The method can be easily modified to solve multireference configuration interaction problems. Performance of the method is demonstrated by several test calculations on systems which require a multireference description. The problem related to the choice of the Fermi-vacuum has also been investigated. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1483856
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  • 3
    Electronic Resource
    Electronic Resource
    Higher excitations in coupled-cluster theory (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2945-2954 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The viability of treating higher excitations in coupled-cluster theory is discussed. An algorithm is presented for solving coupled-cluster (CC) equations which can handle any excitation. Our method combines the formalism of diagrammatic many-body perturbation theory and string-based configuration interaction (CI). CC equations are explicitly put down in terms of antisymmetrized diagrams and a general method is proposed for the factorization of the corresponding algebraic expressions. Contractions between cluster amplitudes and intermediates are evaluated by a string-based algorithm. In contrast to our previous developments [J. Chem. Phys. 113, 1359 (2000)] the operation count of this new method scales roughly as the (2n+2)nd power of the basis set size where n is the highest excitation in the cluster operator. As a by-product we get a completely new CI formalism which is effective for solving both truncated and full CI problems. Generalization for approximate CC models as well as multireference cases is also discussed. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1383290
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  • 4
    Electronic Resource
    Electronic Resource
    Two-body zeroth order Hamiltonians in multireference perturbation theory: The APSG reference state (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 878-890 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A special version of multi-reference perturbation theory is investigated which differs from standard ones by using a zeroth order Hamiltonian that contains two-electron terms explicitly. The method is applicable to reference states that can be written as an antisymmetrized product of two or more electron functions. In that case the zeroth order Hamiltonian has a well defined physical meaning and the matrix elements that come about can be evaluated in an efficient manner. We implemented the theory for the antisymmetrized product of strongly orthogonal geminals wave function and, as a special case, for the generalized valence bond. Illustrative calculations on sample molecules show the reliability of the approach, as well as a significant improvement in many cases compared to MRPT versions based on one-body zeroth order Hamiltonians. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1427918
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  • 5
    Electronic Resource
    Electronic Resource
    Nitrogen bridgehead compounds. 48. Synthesis and stereochemistry of 4-oxo-1,6,7,8,9,9a-hexahydro-4H-pyrido[1,2-a]pyrimidine-3-carboxamides (1985)
    s.l. : American Chemical Society
    The @journal of organic chemistry 50 (1985), S. 2918-2925 
    Details
    ISSN: 1520-6904
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/jo00216a022
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  • 6
    Electronic Resource
    Electronic Resource
    Bond orbital approach for optical rotatory strength calculations (1983)
    Springer
    Theoretical chemistry accounts 63 (1983), S. 43-54 
    Details
    ISSN: 1432-2234
    Keywords: Optical rotatory strength ; Localized orbitals ; Delocalization corrections ; Through space interactions ; Through bond interactions ; Twisted glyoxal
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Directly determined localized approximate molecular Orbitals are used in excitation energy and optical rotatory strength calculations within the CNDO/2 scheme. Using strictly localized bond orbitals one obtains qualitatively good excitation energies, but quantitative agreement can be found only by considering delocalization effects, which have been proved to be crucial in determining the optical rotatory strength. The delocalization interactions are classified as through space and through bond ones and even the latter is found to have significant contributions. The chiroptical properties of the lowest lying transitions in the twisted glyoxal molecule are analysed in terms of localized molecular orbital contributions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00549154
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  • 7
    Electronic Resource
    Electronic Resource
    Delocalization corrections to the strictly localized molecular orbitals: A linearized SCF approximation (1981)
    Springer
    Theoretical chemistry accounts 59 (1981), S. 603-607 
    Details
    ISSN: 1432-2234
    Keywords: Localized orbitals ; Tails of ∼ ; Delocalization corrections to ∼ ; linearized SCF approximation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An explicit formula is derived for calculating the delocalization corrections (tails) to be added to the strictly localized bond orbitals. It was obtained by solving analytically the SCF problem for the interbond interactions in a linearized approximation. The model calculations at the CNDO/2 level show that this simple approach is sufficient to account for the molecular conformations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00552853
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  • 8
    Electronic Resource
    Electronic Resource
    Localization maps by orbital partitioning of the electron density (1993)
    Springer
    Theoretical chemistry accounts 86 (1993), S. 379-389 
    Details
    ISSN: 1432-2234
    Keywords: Localization maps ; Orbital partitioning ; Electron density
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary We define a localization measure for one-determinantal wave-functions based on the partitioning of the total electron density to orbital contributions. The set of occupied orbitals is the more localized the fewer terms are necessary to describe the total density. This measure varies from point to point in space which leads to characteristic localization maps for molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01122430
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  • 9
    Electronic Resource
    Electronic Resource
    The phase diagram of charge- and spin-density waves in polymeric C60 (1997)
    Springer
    Applied physics 64 (1997), S. 315-319 
    Details
    ISSN: 1432-0630
    Keywords: PACS: 61.46.+w; 71.10.+x
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: \chem{(C_{60})_{\chemindex{x}}} at interfullerene distances larger than \approx \valunit{1.5}{\Angstrom} . At smaller ball separation, a special charge-density-wave ground state occurs, which exhibits polarized \chem{C_{60}} spheres. At around experimental bond distances, both CDW and SDW are present with rather small amplitudes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003390050484
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  • 10
    Electronic Resource
    Electronic Resource
    Embedded units in conjugated polymers (1992)
    Springer
    Journal of mathematical chemistry 10 (1992), S. 313-327 
    Details
    ISSN: 1572-8897
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mathematics
    Notes: Abstract The problem of embedding of several monomeric units (benzene, thiophene, isothianaphthene) into linear oligoenes or into a polymer built up by the same units is discussed. Using a simple model Hamiltonian, we evaluate the geometry (bond lengths) and electronic structure (energy gaps) of conjugated oligomers containing up to 200 atoms. Special attention is paid to end effects. The quinoid-aromatic transition and the conjugation interruption due to embedded defects is studied in some detail.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01169180
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