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  • 1
    Electronic Resource
    Electronic Resource
    Hyperpolarizabilities of molecules with frequency dependence and electron correlation (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 3665-3669 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Frequency dependent second hyperpolarizabilities of N2 and the prototype organic molecule trans-butadiene are reported using generalized time dependent Hartree–Fock (TDHF) theory for several frequencies of applied fields. A monotonic increase of the values (positive dispersion) is observed for every nonlinear optical process in a range of frequencies for the applied field. Correlation effects are estimated using a second-order many body perturbation theory and coupled cluster singles and doubles relaxed density method for the analytical determination of the induced dipole moment. Such hybrid results for DC-induced second harmonic generation provide reasonable values in comparison with experiment for N2. However, dispersion and correlation effects in trans-butadiene are both found to be large and could be nonadditive.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460705
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  • 2
    Electronic Resource
    Electronic Resource
    A screened potential molecular-orbital calculation of the π-electron systems of metalloporphin, metallochlorin, and metallobacteriochlorin (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 5045-5052 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The excitation energies and oscillator strengths of the lowest (π,π*) excited states of metalloporphin, -chlorin, and -bacteriochlorin were calculated by the screened potential molecular-orbital theory. The screened potential incorporates instantaneous polarization effects in the molecular Hartree–Fock field. The model reproduced the spectra of the lowest excited states fairly well without use of empirical parameters. The calculated energies of the lowest excited triplets and the Q singlet state were in good agreement with those observed, while the B state was correctly predicted only for the reduced resonance integrals.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452681
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  • 3
    Electronic Resource
    Electronic Resource
    Property evaluation and orbital relaxation in coupled cluster methods (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 502-509 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular electronic properties such as dipole moments, polarizabilities and hyperpolarizabilities and quadrupole moments and polarizabilities, and spin properties such as hyperfine splitting constants and nuclear magnetic coupling constants are predicted by ab initio coupled cluster (CC) methods for a variety of molecules. We compare the results of property evaluation using orbitals that have been allowed to relax in the presence of the perturbation and results obtained using nonrelaxed orbitals. It is demonstrated numerically, and proven formally, that the coupled cluster singles and doubles (CCSD) model using nonrelaxed orbitals is able to include most of the relaxation effects for the evaluation of first- and second-order properties. Thus there is little reason to perform coupled (perturbed) Hartree–Fock calculations as a precursor to correlated CCSD calculations of such properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453596
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  • 4
    Electronic Resource
    Electronic Resource
    Nuclear spin–spin coupling constants evaluated using many body methods (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3945-3949 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In nuclear spin–spin coupling constant determinations, correlation corrections to the Fermi contact term are significant. In this paper we report the coupling constants calculated for the HD and HF molecules obtained by the infinite-order coupled cluster singles and doubles (CCSD) methods and MBPT(4). These are in good agreement with the experimentally estimated value for the Fermi-contact term. In addition, it is well known that the coupled perturbed Hartree–Fock (CPHF) scheme fails for multiply bonded molecules because the closed shell Hartree–Fock solution is triplet unstable. A CCSD method using ordinary nonrelaxed SCF orbitals is presented in order to circumvent this problem, and illustrated by application to the C2H4 molecule. It is shown that CCSD results based upon ordinary SCF orbitals include effectively all the effect of orbital relaxation and reproduce the experimental values for most of the coupling constants. Unlike previous results, the 3J(H–H) constant is positive in agreement with experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450916
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  • 5
    Electronic Resource
    Electronic Resource
    Spin density of radicals by finite field many-body methodsf a@f ) (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 4225-4229 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The spin densities of several small radicals CH3, NH2,CH2CH, BeH, and H2CO+ are calculated by a finite field coupled-cluster method using a spin density operator as a finite perturbation on the systems. The calculated hyperfine coupling constants of the π radicals are in good agreement with experiment at a low level of correlation. The CH2CH radical required a higher level of correlation to obtain the experimental value. The calculations are also performed for the H2CO+ radical where theory has failed to predict the experimental splitting constants. The spin density on hydrogen calculated in the present study is lower than the result obtained by the low temperature gas ESR techniques, although the carbon spin density is well reproduced. An analysis of possible corrections for the H spin density is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448837
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  • 6
    Electronic Resource
    Electronic Resource
    Coupled-cluster calculations of indirect nuclear coupling constants: The importance of non-Fermi contact contributions (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2186-2191 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron correlation effects to the four coupling mechanisms which contribute to the isotropic nuclear spin–spin coupling constant, the Fermi contact (FC), paramagnetic spin–orbit (PSO), spin-dipole (SD), and diamagnetic spin–orbit (DSO) are studied using the equation of motion coupled-cluster (EOM-CC) method. The second-order properties are expressed as a sum-over state (SOS) using EOM-CC intermediate state wave functions. This formulation is simple, accurate, computationally convenient, and involves no truncation. Several molecules, HF, CO, N2, H2O, NH3, and HCl which have been previously shown to have large noncontact contributions are investigated using the EOM-CC method and the results are compared with experiment and other theoretical methods, including polarization propagator and finite-field MBPT(2) methods. Using fairly large basis sets, the EOM-CCSD provides results which agree with experimental indirect nuclear spin–spin coupling constants to within an average error of 13%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467725
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  • 7
    Electronic Resource
    Electronic Resource
    Property evaluation using the Hartree–Fock-density-functional-theory method: An efficient formalism for first- and second-order properties (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7788-7794 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an efficient formalism for property evaluation using the Hartree–Fock-density- functional-theory method. The formalism uses the relaxed density concept for first-order properties which allows us to compute many different components of a property as well as many different properties once we have solved a single set of linear equations spanning the particle–hole space of the system. The density matrix representation of the method indicates that the method accounts for the correlation correction to Hartree–Fock only through particle–hole space. We also show why conventional density functionals based upon a local density and its gradient fail to account for electromagnetic effects. For second-order properties, we show that no extra linear equations need to be solved, once the regular coupled perturbed Hartree–Fock equations are solved.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468272
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  • 8
    Electronic Resource
    Electronic Resource
    Frequency dependent nonlinear optical properties of molecules (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 976-989 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Various nonlinear optical polarizabilities are derived and evaluated by time dependent Hartree–Fock theory (TDHF). The recursive nature of the TDHF theory is exploited to develop formulas that are applicable in any order. The theory is applied to evaluate dispersion effects for the series of molecules CH4, CH3F, CH2F2, CHF3, and CF4. Comparisons are made with results obtained from dc-induced, second-Harmonic generation, and third-Harmonic generation experiments. Additional applications are reported for H2 and HF.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451255
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  • 9
    Electronic Resource
    Electronic Resource
    Hyperpolarizabilities of the hydrogen fluoride molecule: A discrepancy between theory and experiment? (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 2726-2733 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Due to the recent availability of an experimental value for the second- and third-order electric polarizabilities of the HF molecule, possible errors in the previous prediction of these quantities have been considered. These include basis sets, vibrational corrections, frequency dependence, infinite-order correlation corrections, and the effect of triple excitations. Despite the inclusion of all of these effects, the discrepancy between experiment and theory remains. Our best results predict χ(2)(parallel) to be −3.3 to −3.8×10−32 and χ(3)(parallel) to be 45 to 48×10−39 esu which are at best 81% and 69% of the experimental values, respectively. Possible additional corrections are considered with emphasis on basis set completeness. Considering the difficulty in predicting such sensitive quantities, this is rather good agreement. The paper also addresses questions of reliability of ab initio calculations for such high-order properties as hyperpolarizabilities, while identifying some places where the experimental results are susceptible to error.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450348
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular hyperpolarizabilities (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 3022-3037 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report a systematic study of the first and second hyperpolarizabilities of several small molecules at a consistent level of theory and basis sets. Coupled cluster (CC) methods for correlation, analytical high-order time dependent Hartree–Fock (TDHF) theory for dispersion effects, and polarizability-consistent basis sets give agreement to about 10% between the calculated hyperpolarizabilities and the gas phase experiments for the nonpolar molecules, H2, N2, CO2, and C2H4, and effectively nonpolar CO. Results for the polar molecules FH, H2O, NH3, and H2S are improved by adding lone-pair basis functions. For H2O and NH3 there is good (∼10%) agreement with experiment. However, a ∼20% difference between experiment and theory for the FH molecule persists; this difference is discussed in some detail.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464129
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