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  • 1
    Electronic Resource
    Electronic Resource
    Relativistic four-component calculations of indirect nuclear spin–spin couplings in MH4 (M=C, Si, Ge, Sn, Pb) and Pb(CH3)3H (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 3493-3498 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Relativistic four-component random phase approximation (RPA) calculations of indirect nuclear spin–spin coupling constants in MH4 (M=C, Si, Ge, Sn, Pb) and Pb(CH3)3H are presented. The need for tight s-functions also in relativistic four-component calculations is verified and explained, and the effect of omission of (SS–LL) and (SS–SS) two-electron integrals is investigated. Already in GeH4 we see a relativistic increase in the coupling constant by 12%, and for PbH4 the effect is a 156% increase for the one-bond coupling. Large relativistic effects are also computed for the two-bonds couplings. We find that the relativistic effects on the one-bond couplings are mainly due to scalar relativistic factors rather than spin–orbit corrections. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480504
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  • 2
    Electronic Resource
    Electronic Resource
    Static and dynamic polarizability calculations for the polyyne series (C2nH2) with extrapolation to the infinite chain (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2152-2161 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have calculated static and dynamic longitudinal polarizabilities, αzz(−ω;ω), for the polyyne series, C2nH2, using both the uncorrelated random phase approximation and the correlated second-order polarization propagator approximation. The calculated polarizabilities are extrapolated to the value for the infinitely long chain using seven different extrapolation techniques. We employ both conventional schemes, such as the fitting of simple polynomials, as well as new schemes, such as the fitting of a Padé approximant, or purely mathematically motivated nonlinear sequence transformations which have not previously been used in connection with this property. For the direct fits, where the number of parameters is the same as the number of points, we find that the most stable and reliable extrapolation schemes are to be found among the latter. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475594
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  • 3
    Electronic Resource
    Electronic Resource
    Molecular relativistic calculations of the electric field gradients at the nuclei in the hydrogen halides (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 9677-9684 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electric field gradients at the position of the nuclei in the hydrogen halides are calculated using four-component relativistic methods. Benchmark values at the Dirac–Hartree–Fock level of theory are obtained by using large uncontracted basis sets. Electron correlation corrections are obtained by means of finite field MP2, CCSD, and CCSD(T) calculations in smaller basis sets. The importance of spin–orbit coupling and the so-called picture change effect are discussed. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477637
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  • 4
    Electronic Resource
    Electronic Resource
    Effects of the order of the energy asymptotes on the calculations of nuclear magnetic shieldings and static polarizabilities (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 10775-10782 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The second-order polarization propagator approximation (SOPPA) has been applied to the calculation of the nuclear magnetic shielding constants and static polarizabilities of CO, N2, F2, and CH4 in order to investigate the effect of decreasing the order (in perturbation theory) of the poles of the propagator, i.e., the energy asymptotes, but still requiring that all response terms to second order must be included. Our results show that the higher than second-order contributions from the poles are of vital importance for the nuclear magnetic shieldings and of less, but not negligible, importance for the static polarizabilities. As the order of the poles is decreased the SOPPA isotropic shieldings approach the results obtained within second-order perturbation theory (MP2), especially for CO, N2, and F2. This behavior is not as pronounced for the C shielding of CH4 and for polarizabilities. For the shieldings we obtain the best agreement with MP2 (and experiments) when the poles are calculated as Hartree–Fock energy differences using frozen ground state orbitals, and for the polarizabilities when the poles are calculated in the random phase approximation and Tamm–Dancoff approximation. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467890
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  • 5
    Electronic Resource
    Electronic Resource
    A new implementation of the second-order polarization propagator approximation (SOPPA): The excitation spectra of benzene and naphthalene (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 5886-5900 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a new implementation of the second-order polarization propagator approximation (SOPPA) using a direct linear transformation approach, in which the SOPPA equations are solved iteratively. This approach has two important advantages over its predecessors. First, the direct linear transformation allows for more efficient calculations for large two particle–two hole excitation manifolds. Second, the operation count for SOPPA is lowered by one order, to N5. As an application of the new implementation, we calculate the excitation energies and oscillator strengths of the lowest singlet and triplet transitions for benzene and naphthalene. The results compare well with experiment and CASPT2 values, calculated with identical basis sets and molecular geometries. This indicates that SOPPA can provide reliable values for excitation energies and response properties for relatively large molecular systems. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472430
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  • 6
    Electronic Resource
    Electronic Resource
    The calculated electronic excitation spectra of Si2C and Si3 (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 354-360 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The results of second order polarization propagator calculations of the valence excitation energies and oscillator strengths of Si2C and Si3 are reported and compared to the previously published spectrum of SiC2. The character and properties of the excitations are explored and suggestions are made as to excitations which may be amendable to experimental study. A previously observed optical transition in Si clusters is assigned to an intense line in the Si3 spectrum at about 46 000 cm−1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450829
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  • 7
    Electronic Resource
    Electronic Resource
    The structure and spectrum of SiC2 (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 1702-1708 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Second order polarization propagator calculations indicate that the blue/green bands attributed to SiC2 in the atmospheres of certain carbon stars arise from a triangular (C2v) form of the molecule. It appears that this form is separated from a linear (C∞v) form by only a very small barrier, and that spectra from both forms might be observable. The potential for the movement of Si around the C2 fragment, the spectral lines, oscillator strengths, and lifetimes are discussed. We find, as observed previously, that only in a correlated calculation is the triangular geometry the preferred ground state conformation. Also the excitation properties change markedly with correlation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449357
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  • 8
    Electronic Resource
    Electronic Resource
    A coupled cluster polarization propagator method applied to CH+ (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 2112-2118 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new approach to the direct evaluation of excitation energies and transition moments from the polarization propagator is presented. The method, which uses a coupled cluster doubles (CCD) reference state within the framework of perturbative propagator methods, is applied to the lowest singlet and triplet excitations in CH+. Comparison of the coupled cluster doubles polarization propagator approximation (CCDPPA) results with experiments and standard perturbative polarization propagator calculations shows that a significant improvement is obtained with a coupled cluster rather than a Rayleigh–Schrödinger reference state: the singlet excitation energy is improved by about 0.5 eV and the triplet instability of the standard second order approach is removed. The radiative lifetime of the v'=0 level of the A 1Π state is estimated to be very close to 800 ns. The improved performance of the coupled cluster propagator method over propagator calculations based on Rayleigh–Schrödinger expansion mainly stems from a enhancement of the correlation coefficients for the lower double excitations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451156
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  • 9
    Electronic Resource
    Electronic Resource
    Spin–spin coupling constants of CO and N2 (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 2138-2142 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have used the second order polarization propagator method to calculate the indirect nuclear spin–spin coupling constants of 13C–17O and 14N–15N. We have calculated all coupling terms and the vibrationally averaged results are for CO: JFC =7.93 Hz, JSD =−3.99 Hz, JPSO =14.95 Hz, JDSO =0.10 Hz, Jtotal(CO) =18.99 Hz and for N2: JFC =0.82 Hz, JSD =−1.57 Hz, JPSO =3.32 Hz, JDSO =0.03 Hz, and Jtotal(N2) =2.60 Hz. Recent measurements of the two coupling constants gave 1J(13C,17O)=16.4±0.1 Hz and 1J(14N,15N)=1.8±0.6 Hz.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453722
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  • 10
    Electronic Resource
    Electronic Resource
    Quadratic response theory of frequency-dependent first hyperpolarizability. Calculations in the dipole length and mixed-velocity formalisms (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7251-7258 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The quadratic response function (QRF) is evaluated within the random phase approximation (RPA), to compute frequency-dependent first hyperpolarizabilities β(ω,ω). The method treats electron correlation consistent through first order, so the computed values are equivalent to coupled-perturbed Hartree–Fock (CPHF) results. The QRF is obtained by solving systems of linear equations, thus circumventing the RPA eigenvalue problem. The QRF equation of motion is used to develop hyperpolarizability identities in the dipole length and mixed-velocity representations. The two forms of β are equivalent at the RPA level, and provide a useful measure of completeness of basis. The method is applied to the hyperpolarizability of HF and H2O. It is found that basis sets used in previous studies were not saturated for all β components, and that basis sets which satisfy length–velocity sum rules for linear response properties are not sufficient for agreement of quadratic response properties. The calculated dispersion ratios are in good agreement with experimental measurement, indicating that dispersion effects are properly described by frequency-dependent calculations in the RPA at field energies which are small compared to vertical excitation energies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460209
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