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  • 1
    Electronic Resource
    Electronic Resource
    Correlation energies obtained from symmetry-adapted versus symmetry-broken Hartree–Fock solutions in a model delocalized one-dimensional system (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 755-761 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Singlet instabilities of the Hartree–Fock (HF) solutions, leading to bond-centered or atom-centered charge-density waves, occur in extended one-dimensional systems, such as a regular cyclic H4n+2 ring. These solutions are much less delocalized than the symmetry-adapted one, as appears when comparing the equivalent localized molecular orbitals. The symmetry-broken solutions exhibit a finite gap at the Fermi level, whereas for the symmetry-adapted solution the gap can be extrapolated with a 1/N law towards a zero asymptotic value. If one performs post-Hartree–Fock calculations, of Møller–Plesset nth order or linearized coupled cluster type, one obtains a higher final energy from the symmetry-broken HF solutions than from the symmetry-adapted HF wave function. A procedure is proposed to obtain directly symmetry-adapted localized self-consistent field orbitals in systems presenting HF instabilities. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478044
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  • 2
    Electronic Resource
    Electronic Resource
    On the perturbative estimates of the correlation energy from localized orbitals in periodic systems (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 7632-7643 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Starting with self-consistent fields (SCFs), localized orbitals should facilitate the calculation of the correlation energy in extended, and in particular periodic, systems. This idea is exploited on model ring systems (H4n+2). It is shown that for insulating materials [(H2)2n+1, presenting a large gap in the band structure], most of the energy lowering brought by the orders larger than 2 in the canonical many-body perturbation expansion are due to the local-hole–local-particle interaction and that the localized Epstein–Nesbet second-order energies are close to the best correlation-energy estimates. The situation is completely different for small-gap (metalliclike) systems, such as cyclic H4n+2, where the localized second-order approach misses a large fraction of the correlation energy, involving the propagation of the holes and of the particles and implying specific higher-order diagrams. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477385
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  • 3
    Electronic Resource
    Electronic Resource
    State-specific coupled cluster-type dressing of multireference singles and doubles configuration interaction matrix (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 4068-4076 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the theory of state-specific self-consistent intermediate Hamiltonians, one proposes a new dressing of a multireference (MR) singles and doubles configuration interaction (CI) Hamiltonian matrix which insures size consistency. The method is based on a coupled cluster (CC) type factorization of the coefficients of the triples and quadruples and can be considered as leading to a dressed CI formulation of a state-specific MRCC method. Preliminary application of the new procedure to the H4 model and comparison with other MRCC schemes are presented. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471220
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  • 4
    Electronic Resource
    Electronic Resource
    On the origin of size inconsistency of the second-order state-specific effective Hamiltonian method (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 6887-6891 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The deviations of the second-order state-specific effective Hamiltonian method from the strict size consistency are analyzed. Provided that complete or separable model spaces are used, these deviations can be suppressed by a proper choice of nonuniform shifts of energy denominators. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471982
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  • 5
    Electronic Resource
    Electronic Resource
    Self-consistent intermediate Hamiltonians: A coupled cluster type formulation of the singles and doubles configuration interaction matrix dressing (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2576-2588 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper presents a new self-consistent dressing of a singles and doubles configuration interaction matrix which insures size-consistency, separability into closed-shell subsystems if localized molecular orbitals (MOs) are used, and which includes all fourth order corrections. This method yields, among several schemes, a reformulation of the coupled cluster method, including fully the cluster operators of single and double excitations, and partially those of the triples (Bartlett's algorithm named CCSDT-1a). Further improvement can be easily included by adding exclusion principle violating corrections. Since it leads to a matrix diagonalization, the method behaves correctly in case of near degeneracies between the reference determinant and some doubles. Due to its flexibility this formulation offers the possibility of consistent combination with less expensive treatments for the study of very large systems. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469680
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  • 6
    Electronic Resource
    Electronic Resource
    Bonding in C2 and Be2: Broken symmetry and correlation in DFT solutions (1995)
    Springer
    Theoretical chemistry accounts 91 (1995), S. 225-236 
    Details
    ISSN: 1432-2234
    Keywords: Symmetry breaking ; Correlation ; DFI-HF instabilities
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The solution of both Hartree-Fock (HF) and Kohn-Sham (KS) equations is based on the variational principle. Exact wavefunctions would obey the same symmetry restrictions contained in the total hamiltonian. However, the variational principle does not guarantee these symmetry restrictions and the HF and KS solutions are not necessarily symmetric in spin and space. Spatial and spin symmetry broken solutions with lower energies than their restricted analogues are examined for C2 and Be2, in the context of the KS formalism. Comparison with UHF solutions shows that KS instabilities are far less pronounced. The main differences between HF and KS solutions are related to effects of electron correlation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01114989
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  • 7
    Electronic Resource
    Electronic Resource
    Totally dressed SDCI calculations: An application to HF and F2 (1995)
    Springer
    Theoretical chemistry accounts 92 (1995), S. 241-252 
    Details
    ISSN: 1432-2234
    Keywords: Intermediate Hamiltonians ; Total dressing ; Size-consistent configuration interactions ; Linked and unlinked diagrams ; Spectroscopic constants
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary A previously proposed procedure including the linked and unlinked contributions due to Triple and Quadruple excitations into a size-consistent SDCI-like model has been applied to HF and F2 single-bond systems. The procedure is a non-iterative approximation to the more general total dressing model, which is based on the intermediate Hamiltonians theory. Three basis sets have been employed: the correlation consistent cc-pVTZ basis, a similar one including 3d1f polarization functions, and another including one set of g polarization functions. Excellent agreement with experiment and high-quality calculations is obtained for both equilibrium distances and spectroscopic constants. The possibilities of the method in treating single-bond breaking are also demonstrated. Finally, the Linked and Non-Linked contributions from Triple and Quadruple excitations are analysed separately and it is suggested that the addition of the linked triples to the size-consistent SDCI is sufficient to have quantitatively correct spectroscopic properties in going from the size-consistent SDCI to nearly experimental values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01125949
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  • 8
    Electronic Resource
    Electronic Resource
    Bonding in C2 and Be2: Broken symmetry and correlation in DFT solutions (1995)
    Springer
    Theoretica chimica acta 91 (1995), S. 225-236 
    Details
    ISSN: 0040-5744
    Keywords: Key words: Symmetry breaking ; Correlation ; DFI ; HF instabilities
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  The solution of both Hartree–Fock (HF) and Kohn–Sham (KS) equations is based on the variational principle. Exact wavefunctions would obey the same symmetry restrictions contained in the total hamiltonian. However, the variational principle does not guarantee these symmetry restrictions and the HF and KS solutions are not necessarily symmetric in spin and space. Spatial and spin symmetry broken solutions with lower energies than their restricted analogues are examined for C2 and Be2, in the context of the KS formalism. Comparison with UHF solutions shows that KS instabilities are far less pronounced. The main differences between HF and KS solutions are related to effects of electron correlation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01114989
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    Paper (German National Licenses)
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  • 9
    Electronic Resource
    Electronic Resource
    Totally dressed SDCI calculations: An application to HF and F2 (1995)
    Springer
    Theoretica chimica acta 92 (1995), S. 241-252 
    Details
    ISSN: 0040-5744
    Keywords: Key words: Intermediate Hamiltonians ; Total dressing ; Size-consistent configuration interactions ; Linked and unlinked diagrams ; Spectroscopic constants
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  A previously proposed procedure including the linked and unlinked contributions due to Triple and Quadruple excitations into a size-consistent SDCI-like model has been applied to HF and F2 single-bond systems. The procedure is a non-iterative approximation to the more general total dressing model, which is based on the intermediate Hamiltonians theory. Three basis sets have been employed: the correlation consistent cc-pVTZ basis, a similar one including 3d1f polarization functions, and another including one set of g polarization functions. Excellent agreement with experiment and high-quality calculations is obtained for both equilibrium distances and spectroscopic constants. The possibilities of the method in treating single-bond breaking are also demonstrated. Finally, the Linked and Non-Linked contributions from Triple and Quadruple excitations are analysed separately and it is suggested that the addition of the linked triples to the size-consistent SDCI is sufficient to have quantitatively correct spectroscopic properties in going from the size-consistent SDCI to nearly experimental values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01125949
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    Paper (German National Licenses)
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  • 10
    Electronic Resource
    Electronic Resource
    Localized orbitals in nonmetallic ring systems (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 167-180 
    Details
    ISSN: 0020-7608
    Keywords: localized orbitals ; periodic structures ; perturbation theory ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Localized molecular Hartree-Fock orbitals have been determined by means of an iterative procedure consisting of orthogonalization and configuration interaction employing single excitations. For ring systems the rotational symmetry has been included explicitly to obtain Wannier-like orbitals suited for a posteriori correlation calculations using only the most important contributions within a limited region around one reference ring segment. Applications involving different estimates of the correlation energy include as model systems (H2)2n+1, the ionic LinHn, and a weakly bound beryllium ring as well as the strongly covalent molecule (CH2)36 forming a closed ring. In all cases, the localized and canonical MP2 results are close, and the localized Epstein-Nesbet second-order gives a good estimate of more expensive MP4 or CEPA-0 values.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 167-180, 1998
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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