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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 3399-3400 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 7282-7290 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Full fifth-order many-body perturbation theory (MBPT) correlation energy contributions have been calculated for the molecules: BH, CH2 , H2 O, HF, SiH2 , CO, C2 H2, and O3 at the double zeta plus polarization level. The connected T4 component of the fifth-order energy has been computed with only an N6 basis set dependence due to a new factorization introduced for the "connected'' quadruple T4 diagrams. For the systems where MBPT is converging smoothly the fifth-order recovers 30%–50% of the difference between MBPT(4) and the full configuration interaction (FCI) values. The E5 based [2,1] Padé approximants generally provide a better approximation to the FCI energy. The absolute values of E5 corrections vary from 1–2 mhartrees for the hydrides at the equilibrium geometry up to 22 mhartrees for the O3 molecule, which is poorly described by a single self-consistend-field reference function.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 1035-1040 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dipole moment of IF and the remaining five interhalogen halides are computed at the MBPT(4) level employing effective potentials. A comparison between the finite-field and the Hellman–Feynman expectation value approach is presented. The theoretical values are in agreement with the experimental ones to within 10%. In the case of IF, whose dipole moment is unknown, we predict a value of 1.87±0.2 D. The correlation corrections constitute about 20% to 30% of the total dipole moment value, the dominant part of which is introduced at the second order. Only slight differences occur between the Hellman–Feynman expectation value formula and the finite-field procedure for dipole moments at the level of a second-order MBPT wave function.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 2989-2989 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: To study the reliability of the recently developed explicitly correlated coupled cluster method (CCSDT1-R12) we have performed calculations on the four-electron systems Be, Li−, and LiH on various levels of perturbation theory and coupled cluster theory with and without explicit linear rij terms. The convergence of the total energy with increasing size of the basis is much faster than for the conventional coupled cluster approach. Our CCSDT1-R12 energies of −14.667261 Eh for Be and −7.500671 Eh for the Li− ground state are the best ones computed so far and are close to previous estimates of the CCSDT 1 basis set limits. The Be result differs from the "experimental'' nonrelativistic energy by ca. 0.1 mEh, mainly due to neglect of quadruple excitations. Our Born–Oppenheimer energy of LiH at the equilibrium distance of −8.070487 Eh is close to the experimental nonrelativistic energy. The binding energy (D0) of LiH with respect to Li+ and H− is calculated as −7.152 eV, in agreement with the experimental value within a meV. For LiH the harmonic vibrational frequencies and other related spectroscopic constants are studied in their basis dependence as well. The equilibrium distance and the harmonic vibrational frequency of LiH are much less sensitive to the inclusion of terms that explicitly depend on the interelectronic coordinates. Basis set superposition errors are much smaller in the R12 approach than in the conventional calculations, especially for the smaller basis sets. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7738-7762 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: CC-R12—a combination of coupled cluster theory and the R12 method, is presented in which the correlation cusp is treated via inclusion of terms explicitly dependent on the interelectronic distance rij into the exponential expansion of the wave function. A diagrammatic derivation of the CC-R12 equations within the so-called "standard approximation B'' is given at the level of singles, doubles and triples (CCSDT-R12). MBPT(4)-R12 is derived as a byproduct of CCSDT-R12. Fifth order noniterative corrections are also discussed.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6127-6132 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Explicitly correlated coupled cluster (CCSDT-1a-R12) results were obtained for the He2 interatomic potential from a new, integral-direct implementation. With the new code, Gaussian basis sets as large as 11s8p6d5f4g3h could be employed, and the potential energy curve was calculated over a wide range using a basis of the type 11s8p6d5f4g.This curve is very close to represent the basis set limit of the CCSDT-1a approach. At the internuclear separation R=5.6 a0, the CCSDT-1a limiting value for the interaction energy is −10.68 K. As the effect of quadruple substitutions can be estimated as −0.32 K, this limiting value is perfectly consistent with the accurate quantum Monte Carlo calculation of Anderson et al. [J. Chem. Phys. 99, 345 (1993)], who reported a well depth of −11.01±0.10 K. On the other hand, however, CCSDT-1a-R12 calculations of the He2 potential energy curve strongly indicate that the most recent semiempirical potentials available in the literature are slightly too repulsive for short (R≤4.0 a0) interatomic distances. © 1995 American Institute of Physics.
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  • 8
    ISSN: 1432-2234
    Keywords: Key words: Coupled-cluster theory ; Explicitly correlated ; Electric properties ; Hyperpolarizability ; Basis set limit
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. Explicitly correlated MBPT-R12 and coupled cluster [up to CCSD(T)-R12] methods have been used in calculations of various (vibrationless) electrical properties for the LiH molecule, including the dipole and quadrupole moments, dipole and quadrupole polarizability tensors, dipole hyperpolarizability tensors, and the second dipole hyperpolarizability tensors. Generally, with extension of the basis set the R12 method did not lead to faster convergence for the calculated properties towards the basis limit. Nevertheless, R12 calculations serve as useful indicators to judge the reliability of the results, and substantially help in determining the accuracy. Results obtained with the 11s8p6d5f/9s8p6d5f basis and CCSD(T)-R12 calculated within this work should be close to the basis set limit.
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  • 9
    ISSN: 1432-2234
    Keywords: MB-RSPT calculations ; He2, interaction energy of ∼
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The efficiency of the MB-RSPT in the calculations of the correlation contributions to the interaction energies was investigated, using He2 as a model Van der Waals system. The attention has been focused on the convergency of the perturbation expansion in the calculations of the interaction energy and on the analysis of the fourth-order terms of MB-RSPT. The rôle of the renormalization term in the correct long-range behaviour of the interaction potential has been emphasized.
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  • 10
    ISSN: 1432-2234
    Keywords: Coupled cluster method ; One-electron properties ; Expectation value
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The ability of various approximate coupled cluster (CC) methods to provide accurate first-order one-electron properties calculated as expectation values is theoretically analysed and computationally examined for BH and CO. For actual calculations the infinite number of terms of the expectation value expansion (〈O〉=〈φ¦exp (T +)O exp (T)¦φ〉c) was truncated so that T 1 T 2, T 3, and (1/2) T 2T2 clusters were retained on both sides of O. The role of individual clusters is carefully discussed. Inclusion of T 1, is unavoidable, but if triples are essential in the energy evaluation, they may play an even more important role in the property expansion, as shown in the case of CO. It is shown that the CC wave function, which is exact to second order, effectively satisfies the Hellmann-Feynman theorem.
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