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  • 1
    Electronic Resource
    Electronic Resource
    An explicitly correlated coupled cluster calculation of the helium–helium interatomic potential (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6127-6132 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470440
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  • 2
    Electronic Resource
    Electronic Resource
    Coupled cluster theory that takes care of the correlation cusp by inclusion of linear terms in the interelectronic coordinates (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7738-7762 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468266
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  • 3
    Electronic Resource
    Electronic Resource
    Dipole moment of IF and other interhalogen molecules (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 1035-1040 
    Details
    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
    URL: http://dx.doi.org/10.1063/1.454271
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  • 4
    Electronic Resource
    Electronic Resource
    The accuracy of atomization energies from explicitly correlated coupled-cluster calculations (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2022-2032 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The accuracy of atomization energies obtained from explicitly correlated coupled-cluster R12 calculations (CC-R12)—including single and double excitation operators (CCSD-R12) and a posteriori perturbative corrections for triple excitations [CCSD[T]-R12 and CCSD(T)-R12]—is studied for CH2(1A1), NH3, H2O, HF, N2, CO, and F2. The basis-set convergence with functions of high angular momentum is demonstrated. Unlike for conventional calculations, already the spdf saturation on nonhydrogen atoms and spd saturation on hydrogen are sufficient for CC-R12 calculations to provide results accurate to within 1 kJ/mol of the limit of a complete basis. Remaining small uncertainties at the CCSD[T]-R12 or CCSD(T)-R12 levels are attributed to the insufficient convergence within the coupled-cluster hierarchy towards the limit of full configuration interaction. It is shown that near the basis-set limit (as provided by CC-R12 calculations) the CCSD[T] variant of the triples correction gives, on average, results closer to the experimental data than its CCSD(T) counterpart. Approximate error bars are estimated by one single CC-R12 calculation from the difference between the CCSD[T] and CCSD(T) methods and from the second-order electronic cusp correction in standard approximation B. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1384011
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  • 5
    Electronic Resource
    Electronic Resource
    The full CCSDT model for molecular electronic structure (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 7041-7050 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The full coupled-cluster model (CCSDT) single, double, and triple excitation method defined by the wave function exp(T1+T2+T3)||Φ0〉 is formulated and computationally implemented for the first time. Explicit computational equations are presented. The method is applied to numerous examples including BH, FH, C2H2, CO, Ne, F−, and H2O to assess its applicability to the correlation problem. Results from CCSDT agree with full CI, to an average error of less than 1 kcal/mol even for difficult bond breaking examples. A series of results for various approximate, but computationally more efficient versions of the full CCSDT model are also studied and shown to give results in excellent agreement with CCSDT. Additional comparisons with fifth-order MBPT are reported.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452353
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  • 6
    Electronic Resource
    Electronic Resource
    Erratum: "The accuracy of atomization energies from explicitly correlated coupled-cluster calculations" [J. Chem. Phys. 115, 2022 (2001)] (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 2989-2989 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1491401
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  • 7
    Electronic Resource
    Electronic Resource
    Erratum: "The accuracy of atomization energies from explicitly correlated coupled-cluster calculations" [J. Chem. Phys. 115, 2022 (2001)] (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 5690-5690 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1401140
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  • 8
    Electronic Resource
    Electronic Resource
    Use of molecular symmetry in coupled-cluster theory (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 411-415 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown that intermediate arrays resulting from the quasilinearization of coupled-cluster theory with double excitations, and having the form of pseudointegrals 〈ab||ij〉n, retain symmetry properties of the ordinary two-electron integrals 〈ab||ij〉. The 〈ab||ij〉n elements vanishing on symmetry grounds may be eliminated a priori, as well as contributions to nonvanishing 〈ab||ij〉n pseudointegrals from vanishing 〈ab||ij〉n−1 or 〈ab||ij〉 terms. A program using molecular symmetry in this way has been coded and its speed compared with that of gaussian 82.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453585
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  • 9
    Electronic Resource
    Electronic Resource
    Towards a full CCSDT model for electron correlation (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 4041-4046 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Coupled cluster models for electron correlation which include the effects of single, double, and triple excitation operators are analyzed. An alternate version of the approximate CCSDT-1 method is implemented. In this version, the full CCSDT cluster operator eT1+T2+T3 is preserved in the creation of single and double excitation coefficients, but in calculation of triple excitation coefficients only the T2 operator is used. We also present a theoretical analysis of the simplest improvement for the evaluation of the contribution of triples beyond that obtained with fourth-order MBPT. In this approximation, an MBPT(4)-like calculation of the triples energy is evaluated with converged CCSD T2 coefficients. This is found to offer a good approximation to the converged CCSDT-1 results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449067
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  • 10
    Electronic Resource
    Electronic Resource
    Fifth-order many-body perturbation theory for molecular correlation energies (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 7282-7290 
    Details
    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
    URL: http://dx.doi.org/10.1063/1.456206
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