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  • 1
    Electronic Resource
    Electronic Resource
    Theory of hyperfine coupling constants of solvated molecules: Applications involving methyl and ClO2 radicals in different solvents (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 629-635 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A multiconfiguration self-consistent reaction field method is presented for calculating isotropic hyperfine coupling constants of solvated molecules. The solvent model is based on generalizations of Kirkwood's model for describing solvation effects. Isotropic hyperfine coupling constants are calculated for the methyl and ClO2 radicals in different solvents. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470858
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  • 2
    Electronic Resource
    Electronic Resource
    The integral-direct coupled cluster singles and doubles model (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 4157-4165 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An efficient and highly vectorized implementation of the coupled cluster singles and doubles (CCSD) model using a direct atomic integral technique is presented. The minimal number of n6 processes has been implemented for the most time consuming terms and point group symmetry is used to further reduce operation counts and memory requirements. The significantly increased application range of the CCSD method is illustrated with sample calculations on several systems with more than 500 basis functions. Furthermore, we present the basic trends of an open ended algorithm and discuss the use of integral prescreening. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471227
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  • 3
    Electronic Resource
    Electronic Resource
    Response functions in the CC3 iterative triple excitation model (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 7429-7441 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The derivation of response functions for coupled cluster models is discussed in a context where approximations can be introduced in the coupled cluster equations. The linear response function is derived for the approximate coupled cluster singles, doubles, and triples model CC3. The linear response functions for the approximate triples models, CCSDT-1a and CCSDT-1b, are obtained as simplifications to the CC3 linear response function. The consequences of these simplifications are discussed for the evaluation of molecular properties, in particular, for excitation energies. Excitation energies obtained from the linear response eigenvalue equation are analyzed in orders of the fluctuation potential. Double replacement dominated excitations are correct through second order in all the triples models mentioned, whereas they are only correct to first order in the coupled cluster singles and doubles model (CCSD). Single replacement dominated excitation energies are correct through third order in CC3, while in CCSDT-1a, CCSDT-1b, and CCSD they are only correct through second order. Calculations of excitation energies are reported for CH+, N2, and C2H4 to illustrate the accuracy that can be obtained in the various triples models. The CH+ results are compared to full configuration interaction results, the C2H4 results are compared with complete active space second order perturbation theory (CASPT2) and experiment, and the N2 results are compared to experiment. Double replacement dominated excitations are improved significantly relative to CCSD in all the triples models mentioned, and is of the same quality in CC3 and CCSDT-1a. The single replacement dominated excitation are close to full configuration interaction results for the CC3 model and significantly improved relative to CCSD. The CCSDT-1 results for the single replacement dominated excitations are not improved compared to CCSD. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470315
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  • 4
    Electronic Resource
    Electronic Resource
    Perturbative triple excitation corrections to coupled cluster singles and doubles excitation energies (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 1451-1459 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The contributions from various excitation levels to excitation energies calculated within a coupled cluster framework are analyzed in terms of order in the fluctuation potential. In particular, the role of triple excitations is considered, focusing on their importance for describing excitations of single and double replacement dominated character. Several noniterative triples corrections to the coupled cluster singles and doubles (CCSD) excitation energies are proposed. In the CCSDR(3) approach, which is a noniterative analog to the recently proposed iterative CC3 model, single replacement dominated excitations are correct through third order in the fluctuation potential, and double replacement dominated excitations are correct through second order. The performance of CCSDR(3) is compared to other noniterative and iterative triples models in benchmark calculations on CH+, Ne, BH, and CH2. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472007
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  • 5
    Electronic Resource
    Electronic Resource
    Large-scale calculations of excitation energies in coupled cluster theory: The singlet excited states of benzene (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 6921-6939 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Algorithms for calculating singlet excitation energies in the coupled cluster singles and doubles (CCSD) model are discussed and an implementation of an atomic-integral direct algorithm is presented. Each excitation energy is calculated at a cost comparable to that of the CCSD ground-state energy. Singlet excitation energies are calculated for benzene using up to 432 basis functions. Basis-set effects of the order of 0.2 eV are observed when the basis is increased from augmented polarized valence double-zeta (aug-cc-pVDZ) to augmented polarized valence triple-zeta (aug-cc-pVTZ) quality. The correlation problem is examined by performing calculations in the hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3, as well as by using the CCSDR(3) perturbative triples corrections. The effect of triple excitations are less than 0.2 eV for all excitations except for the 2 1E2g state. The calculated excitation energies are compared with experiment and other theoretical results. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471985
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  • 6
    Electronic Resource
    Electronic Resource
    Surprising cases of divergent behavior in Møller–Plesset perturbation theory (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 5082-5090 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High-order Møller–Plesset perturbation calculations have been carried out for several small molecules and compared to full configuration interaction (FCI) results. The convergence of the Møller–Plesset series is found to depend crucially on the one-electron basis sets. Addition of diffuse basis functions leads in some cases to divergent behavior of the Møller–Plesset series, even for highly single reference dominated systems as Ne and HF. The results thus questions the usefulness of higher-order perturbation calculations as a vehicle for obtaining arbitrary accuracy of quantum chemical calculations and raises the fundamental theoretical question: When does Møller–Plesset perturbation theory converge for many-electron systems in extended basis sets? © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472352
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  • 7
    Electronic Resource
    Electronic Resource
    A theoretical study of the electronic spectrum of water (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 8101-8112 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic spectrum of water is investigated using coupled cluster electronic structure methods. Vertical excitation energies are calculated for both gas phase H2O, various water clusters, as well as from models designed to simulate condensed phase effects. Four different approaches for describing condensed phase effects on the electronic transitions are investigated: continuum (a single water molecule embedded in a dielectric medium), discrete (water clusters), semidiscrete (a water pentamer cluster embedded in a dielectric medium), and intermolecular perturbation methods. The results are compared with experimental results. The solvent shift on the lowest state appears to be reasonably described by discrete and semidiscrete methods. It is very difficult to model the condensed-phase effects for the higher states of the pure liquid. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1316035
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  • 8
    Electronic Resource
    Electronic Resource
    A coupled-cluster solvent reaction field method (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 1365-1375 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method for introducing the coupling between a molecule and an outer solvent in molecular coupled-cluster calculations is presented. The method is a coupled-cluster self-consistent reaction field (CCSCRF) method. The outer solvent is represented as a dielectric medium which is characterized by the dielectric constants of the solvent. The interaction between solute and solvent is introduced into the coupled-cluster Lagrangian and a self-consistent solution is obtained. Sample calculations are presented for total energies and structures for H2O, H2S, and CO. Furthermore, the rotation barrier of 1,2-Dichloroethane is determined in vacuum and different solvents and the results are compared to experimental data. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478026
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  • 9
    Electronic Resource
    Electronic Resource
    Coupled cluster response theory for solvated molecules in equilibrium and nonequilibrium solvation (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 8348-8360 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown how response functions can be derived as derivatives of a time-averaged quasienergy for molecules surrounded by a polarizable dielectric continuum. The vacuum quasienergy is modified with additional solvent terms which depend on the chosen solvent configuration. We consider equilibrium and nonequilibrium solvent models. Coupled cluster response functions are derived by introducing a corresponding coupled cluster quasienergy variational Lagrangian. Computational tractable expressions are derived for linear, quadratic, and cubic response functions. Sample calculations are reported for solvation effects on excitation energies, oscillator strengths, dipole moments, and frequency-dependent polarizabilities of H2S and furan. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478745
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  • 10
    Electronic Resource
    Electronic Resource
    Polarizabilities of CO, N2, HF, Ne, BH, and CH+ from ab initio calculations: Systematic studies of electron correlation, basis set errors, and vibrational contributions (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 4745-4757 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dipole-polarizabilities of CO, N2, HF, Ne, BH, and CH+ have been investigated using coupled-cluster methods. An extensive basis set study has been carried out and electron correlation effects have been studied employing a hierarchy of coupled-cluster models. Vibrational-averaged electronic polarizabilities as well as the pure vibrational polarizabilities have been calculated based on ab initio energy and property curves. The frequency-dependency of the vibrationally-averaged electronic polarizabilities and vibrational polarizabilities are discussed. The final results are compared with available experimental data. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477086
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