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  • 1
    Digitale Medien
    Digitale Medien
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 96 (1992), S. 2162-2171 
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 998-1008 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Besides unavoidable discontinuities of the finite-order potential curves derivatives at the symmetry-breaking point, the UHH Möller–Plesset perturbation series show very slow convergence at intermediate and large interatomic distances. This bad behavior has two origins: (i) the MP denominators tend towards a constant value instead of vanishing so that each contribution is decreasing too rapidly with the interatomic distance; this remark condemns the use of MP zeroth-order Hamiltonian and forces to adopt an Epstein–Nesbet definition; (ii) the coupling between the singly and doubly excited determinants is very strong at intermediate distances, and the effect of the singly excited configurations, which only appears at the fourth order, slows down the convergence rate. These trends are both analytically demonstrated and numerically illustrated in the model minimal basis sets He++2 and N2 problems. A proposal to include the effect of singly excited determinants through a low-dimensional effective intermediate-type Hamiltonian has significantly improved the second-order results. By the way it is demonstrated that the perturbation from a singlet symmetry-broken HF solution (charge instability) necessarily diverges at intermediate distance.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3163-3173 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Simple molecular orbital (MO) arguments as well as valence bond analysis of accurate MO–CI wave functions discriminate between essentially neutral and essentially ionic eigenstates, which also appear clearly from careful studies of the Hubbard bands. While homoatomic molecules have a neutral ground state, Lin clusters for n=4–8 have ionic states in the sense of the VB theory, with large static or dynamic charge displacements. All these structures involve odd-membered rings. Secondary minima of neutral VB character appear to be built from four-membered rings and have large components on spin waves. This analysis supports a qualitative difference between chemical bonding in nonpolar systems with normal valency of the atoms and the metallic bonding (involving hypervalent atoms), which cannot be regarded as resonating covalent bonds. The role of the β/U ratio (i.e., delocalization/correlation) and of dimensionality is discussed (one-dimensional clusters would remain neutral).
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 433-440 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Ab initio CI calculations have been performed for the HCl− system, using moderately large basis sets. For large interactomic distances, the potential curves of the three bound states (two 2∑+ and one 2Π) compare favorably with those previously computed with very large basis sets. The extension of a recently published procedure yields nearly diabatic potential curves for the unbound region, and enables to discriminate between valence-like resonant states, of essentially repulsive character and diffuse states, the potential curves of which parallel one of the neutral molecule curves; the former are nearly basis-set independent while the latter are sensitive to the basis. Estimations of the widths are also given. For the HCl equilibrium distance, the first 2∑+ "resonant state'' is located at 2.3 eV above the v=0 level of HCl, with a large width of 2 eV; the second 2∑+ and 2Π resonant states are located at 9.4 and 7.1 eV, respectively, with widths of 2.5 and 1.2 eV.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 6887-6891 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    s.l. : American Chemical Society
    Journal of the American Chemical Society 96 (1974), S. 3393-3397 
    ISSN: 1520-5126
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2576-2588 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 4068-4076 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
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  • 9
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2205-2212 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A general strategy for the calculation of energy differences between states of different total spin is proposed. The procedure goes through the definition of a minimal model space and a set of energy-difference contributing determinants is established through the second-order development of the corresponding Hamiltonian in the framework of the quasidegenerate perturbation theory. The forementioned determinants are treated variationally in a so-called differential configuration interaction (CI). Several test calculations for simple model Li clusters have been carried out, and the results of the differential CI compare favorably with those of the exact solution in the full CI procedure.
    Materialart: Digitale Medien
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  • 10
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 1440-1449 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Intermediate Hamiltonians are effective Hamiltonians which are defined on an N-dimensional model space but which only provide n〈N exact eigenvalues and the projections of the corresponding eigenvectors onto the model space. For a single root research, the intermediate Hamiltonian may be obtained from the restriction of the Hamiltonian to the model space by an appropriate, uniquely defined dressing of the diagonal energies or of the first column. Approximate self-consistent dressings may be proposed. The simplest perturbative form gives the same result as the original 2nd order intermediate Hamiltonian or the "shifted Bk'' technique but it is of easier implementation. Self-consistent inclusion of higher order exclusion principle violating corrections greatly improves the results, especially for nearly degenerate problems, as shown on several illustrative applications. Possible generalizations to enlarged or reduced model spaces are discussed.
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