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  • 1
    Electronic Resource
    Electronic Resource
    Size-extensivity correction for the state-specific multireference Brillouin–Wigner coupled-cluster theory (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 8779-8784 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a simple a posteriori correction for the state-specific multireference Brillouin–Wigner coupled-cluster (MR BWCCSD) theory, which eliminates its size-extensivity error. In the converged amplitudes we drop terms that were identified to be responsible for the lack of size extensivity. We performed MR BWCCSD calculations with this correction on CH2, SiH2, twisted ethylene, F2, and ozone that are all, from the computational point of view, typical representatives of two-reference problems. Comparison with rigorously size-extensive calculations and experiment shows that the size-extensivity error of the corrected MR BWCCSD is only a few tenths of kcal/mol. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481493
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  • 2
    Electronic Resource
    Electronic Resource
    Spin adapted restricted Hartree–Fock reference coupled cluster theory for open shell systems (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 3706-3716 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The coupled clusters (CC) method for effective calculations of open shell systems with the single restricted Hartree–Fock (ROHF) reference determinants is formulated. We apply the spin adaptation described in our previous work, aimed at removing the spin contamination in both coupled cluster (CC) amplitudes and CC energy, both for the linear and nonlinear versions of the single and double excitation coupled cluster (CCSD) method. We give a comparison of results with adapted and nonadapted methods. Together with the elimination of the spin contamination, our spin adaptation yields CC equations, which reduce the number of CC amplitudes and the number of arithmetical operations, which results in computational time comparable with analogous closed shell calculations. The complication in the full spin adapted CC method is that individual spin states require specific spin adapted excitations, so that for every spin state, a different implementation is needed. It is possible to define a very effective approximation to the full spin adapted model that is common to all half-spin states. The method is less complicated in implementation, while the computational requirements increase in comparison with the full spin adapted method only slightly. We also suggest how to introduce spin adaptation in a very simple way to existing ROHF CCSD programs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466359
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  • 3
    Electronic Resource
    Electronic Resource
    Quasidegenerate many-body perturbation theory with self-consistently adapted core reference state (SCA QD MBPT). I. Theoretical formulation and its application to the potential energy curve of the ground state of Li2 (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 3260-3270 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The alternative version of Brandow's QD MBPT is formulated, in which the core vacuum state is redefined with respect to a chosen reference state from model space. This is done through a one-particle operator U in such a way that the total electronic Hamiltonian is not changed. The second order of SCA QD MBPT is applied to the study of the potential energy curve of the ground state of Li2. The results are compared with both the highly sophisticated ab initio calculations (MC SCF, CI-SDT) and the experimental results. The convergency properties of the SCA QD MBPT expansion are discussed with respect to a chosen one-determinantal zeroth-order wave function as well as to an applied model space.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450256
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  • 4
    Electronic Resource
    Electronic Resource
    Single-root multireference Brillouin-Wigner coupled-cluster theory: Applicability to the F2 molecule (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 6571-6579 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently developed single-root multireference Brillouin-Wigner coupled-cluster (MR BWCC) theory, which deals with one state at a time while employing a multiconfigurational reference wave function, is applied to the ground state of the F2 molecule using a two-determinant reference space at the level of the CCSD approximation. The method represents a brand-new coupled-cluster (CC) approach to quasidegenerate problems which combines merits of two theories: the single-reference CC method in a nondegenerate case and the Hilbert space MR CC method in quasidegenerate case. The method is able to switch itself from a nondegenerate to a fully degenerate case in a continuous manner, providing thus smooth potential energy surfaces. Moreover, in contrast to the Hilbert space MR CC approaches, it does not contain the so-called coupling terms and completely reduces to the standard single-reference CC method in a highly nondegenerate region. Using a [4s,3p,1d] and [4s,3p,2d,1f ] basis sets, the calculated potential energy curves are smooth, dissociate correctly and the results are compared with other available multireference techniques as well as experiment. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476071
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  • 5
    Electronic Resource
    Electronic Resource
    Spin adapted restricted open shell coupled cluster theory. Linear version (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 5074-5080 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe the linear version of the spin adapted coupled cluster theory with all single and double excitations for open shell systems in restricted Hartree–Fock formalism. The method is based on the partitioning of the Hamiltonian developed by Hubac and Cársky [I. Hubac and P. Cársky, Phys. Rev. A 22, 2392 (1980)] in restricted Roothaan open shell formalism. Correlation energy of some simple molecular systems is calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463828
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  • 6
    Electronic Resource
    Electronic Resource
    Calculation of vertical ionization potentials of H2O and Ne by many-body Rayleigh-Schrödinger perturbation theory (1977)
    Springer
    Theoretical chemistry accounts 45 (1977), S. 185-195 
    Details
    ISSN: 1432-2234
    Keywords: Ionization potentials ; Many-body perturbation theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The vertical ionization potentials of H2O and Ne are calculated by many-body Rayleigh-Schrödinger perturbation theory up to third order. The comparison of the present method with the other approaches is done.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00552680
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  • 7
    Electronic Resource
    Electronic Resource
    Calculation of vertical ionization potentials of H2O and Ne by many-body Rayleigh-Schrödinger perturbation theory (1977)
    Springer
    Theoretical chemistry accounts 45 (1977), S. 185-195 
    Details
    ISSN: 1432-2234
    Keywords: Ionization potentials ; Many-body perturbation theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The vertical ionization potentials of H2O and Ne are calculated by many-body Rayleigh-Schrödinger perturbation theory up to third order. The comparison of the present method with the other approaches is done.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02401399
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  • 8
    Electronic Resource
    Electronic Resource
    Restricted Hartree-Fock and unrestricted Hartree-Fock as reference states in many-body perturbation theory: a critical comparison of the two approaches (1991)
    Springer
    Theoretical chemistry accounts 80 (1991), S. 407-425 
    Details
    ISSN: 1432-2234
    Keywords: Many-body perturbation theory ; Hartree-Fock ; Potential surfaces ; Open-shell systems
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The paper deals with two topics related to the problem which reference state is better for many-body perturbation theory: restricted Hartree-Fock (RHF) or unrestricted Hartree-Fock (UHF)? The first topic concerns the potential surfaces. Several examples are presented to show shortcomings of the two approaches and a simple way is presented which seems to give a useful potential curve in the whole range of interatomic distances by a composition of RHF and UHF potential curves. The second topic concerns the many-body perturbation theory for open-shell systems in the RHF formalism. The method is critically examined and compared with the ordinary many-body perturbation theory using UHF as the reference. This examination of many-body techniques provides also some insight into the problems inherent of the SCF theory: spin contamination from higher multiplets, localization of orbitals, and self-consistency effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01117420
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  • 9
    Electronic Resource
    Electronic Resource
    Many-Body Rayleigh-Schrödinger Perturbation calculations of the correlation energy of open shell molecules in the restricted Roothaan-Hartree-Fock formalism. Application to heats of reaction and energies of activation (1980)
    Springer
    Theoretical chemistry accounts 56 (1980), S. 315-328 
    Details
    ISSN: 1432-2234
    Keywords: Correlation energy of radicals ; Perturbation calculations of correlation energy of radicals
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The title technique was applied to a series of elementary chemical reactions. Second and third order contributions to the correlation energy were computed for the basis sets of the double zeta and double zeta plus polarization quality. Calculated heats of reaction and energies of activation were compared with the experimental data and the results of the bestab initio calculations reported in the literature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00552595
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  • 10
    Electronic Resource
    Electronic Resource
    Correlation energies in open shell systems. Comparison of CEPA, PNO-CI and perturbation treatments based on the restricted Roothaan-Hartree-Fock formalism (1982)
    Springer
    Theoretical chemistry accounts 60 (1982), S. 445-450 
    Details
    ISSN: 1432-2234
    Keywords: Correlation energy of radicals ; Many-body perturbation theory ; CEPA
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A set of simple molecules in closed and open-shell ground states is treated by the three techniques mentioned in the title, using the same geometries and basis sets (DZ + P). It is found that for nearly all molecules treated in this study (exceptions are H2 and CH3) consistently about 98% of the CEPA valence shell correlation energy is obtained by third-order many-body Ray-leigh-Schrödinger perturbation theory (MB-RSPT). The CEPA and MB-RSPT results for reaction energies and barrier heights for some simple reactions differ by 0 to 30 kJ/mol, the CEPA results being in most cases closer to experiment than MB-RSPT, while CI results are much less reliable as long as CI is limited to singly and doubly substituted configurations only.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00548697
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