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  • American Institute of Physics (AIP)  (151)
  • Molecular Diversity Preservation International (MDPI)
  • 1
    Electronic Resource
    Electronic Resource
    The dipole moment of carbon monoxide (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 6660-6663 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dipole moment of CO has been calculated with many-body perturbation theory (MBPT) and coupled cluster (CC) methods using basis sets which have been optimized at the MBPT-2 level. It is demonstrated that triple excitations as well as g-type functions in the basis set are crucial to obtain satisfactory agreement with experiment. The most reliable prediction (0.125 D) is obtained at the CCSD(T) (coupled cluster including all single, double, and connected triple excitations, perturbatively) level of theory using a 10s9p4d2f1g basis set (160 basis functions). This result is in excellent agreement with the experimental value of 0.122 D.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460293
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  • 2
    Electronic Resource
    Electronic Resource
    The vibrational frequencies of ozone (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 489-494 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equilibrium structure and harmonic frequencies of ozone have been determined at various levels of coupled-cluster theory using large one-particle basis sets. We have used the singles and doubles coupled-cluster (CCSD) method, the recently proposed CCSD(T) method, and the CCSDT-1 method. The largest one-particle basis sets utilize the atomic natural orbital (ANO) general contraction scheme and contain up through g-type functions. It is shown that f-type functions are essential for obtaining quantitative accuracy in the equilibrium structure and the harmonic frequencies (i.e.,±0.003 A(ring) and ± 30 cm−1). The CCSD(T) level of theory in conjunction with a [4s 3p 2d 1f] ANO basis set is found to yield an equilibrium geometry and harmonic frequencies in excellent agreement with experiment. At this level of theory the harmonic frequencies are ω1(a1)=1152, ω2(a)1=717, and ω3(b2)=1057 cm−1 to be compared with the experimental values of 1135, 716, and 1089 cm−1, respectively. This success encouraged us to determine the cubic and quartic force constants and evaluate the anharmonic corrections to the fundamental vibrational frequencies. This level of theory is shown to be quite good for the a1 vibrational modes, but some of the cubic force constants involving ω3(b2) are shown to have large errors leading to an anharmonic correction for the asymmetric stretch that is about a factor of 2 too large.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459548
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  • 3
    Electronic Resource
    Electronic Resource
    An assessment for the full coupled cluster method including all single, double, and triple excitations: The diatomic molecules LiH, Li2, BH, LiF, C2, BeO, CN+, BF, NO+, and F2 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 568-573 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The full coupled cluster singles, doubles, and triples (CCSDT) model for molecular electronic structure theory has been used in conjunction with two less complete coupled cluster methods (CCSD, CCSDT-1) to predict the equilibrium bond distances and harmonic vibrational frequencies of a family of closed-shell ground state diatomic molecules. The results are compared with the configuration interaction method including all single, double, triple, and quadruple excitations (CISDTQ) and experiment. With previous CCSDT results for HF, OH−, N2, and CO, the present work provides a realistic assessment of the CCSDT method. The BeO molecule in particular places great demands on the theory, and the linearized triple excitation model CCSDT-1 fails badly.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458407
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  • 4
    Electronic Resource
    Electronic Resource
    A coupled cluster study of As2 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6722-6727 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical calculations at the self-consistent field (SCF) and coupled-cluster method including all single and double excitations (CCSD) have been carried out for As2. A variety of basis sets employing both segmented contractions and atomic natural orbitals have been developed and investigated. The predicted equilibrium bond length (re=2.107 A(ring)), harmonic vibrational frequency (ωe=447 cm−1), and ionization potentials (9.75, 10.08, and 14.78 eV) are in excellent agreement with experimental values 2.103 A(ring), 430 cm−1, and 9.89 eV, 10.22 eV, and 15.32 eV; respectively. The best theoretical estimates for the dissociation energy (71.6 kcal/mol) and for the electron affinity (0.56 eV) were obtained at the CCSD level of theory employing large basis sets. However, given the importance of correlation effects on these two properties (the corresponding SCF values are 12.4 kcal/mol and 0.42 eV), the theoretical predictions could substantially be affected by the inclusion of connected triple excitations not considered in this work. The open-shell atomic As(4S) and diatomic calculations for As+2 and As−2 were carried out using a recently developed CCSD program that is based on restricted Hartree–Fock references and does not include any spin contamination for the correlation energies. The importance of correlating 3d electrons and the basis-set superposition error for De are analyzed. The effect of including d- and f-type polarization functions in the basis set is also discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458307
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  • 5
    Electronic Resource
    Electronic Resource
    The photodissociation of formaldehyde: A coupled cluster study including connected triple excitations of the transition state barrier height for H2CO→H2+CO (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 3629-3636 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio molecular quantum mechanics has been applied to the unimolecular dissociation of H2CO. Basis sets as large as triple zeta plus double polarization (TZ+2P) were used in conjunction with complete optimization of all stationary point geometries. The classical barrier height is predicted with the TZ+2P basis set to be 101.9 (SCF), 95.0 (CISD), 90.4 (CCSD), and 86.8 kcal/mol (CCSDT-1). With correction for zero-point vibrational energies, the activation energy is predicted to be 81.4 kcal/mol, in good agreement with experimental estimates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455821
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  • 6
    Electronic Resource
    Electronic Resource
    The nuclear quadrupole moment of 14N. A theoretical prediction from full valence shell and full configuration interaction atomic wave functions (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 4020-4024 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Full configuration interaction ab initio studies of the 2p 3p 1P1 state of N+ are carried out in order to obtain the molecular electric field gradient at the nitrogen nucleus. A careful calibration of the basis set leads to a q value (−0.94 a.u.) obtained using a 〈13s9p5d2f/10s7p5d2f〉 basis set with an estimated accuracy of ±0.01 a.u. Combined with the experimental nuclear quadrupole coupling constant for this system, we predict the nuclear quadrupole moment of 14N to be Q(14N)=(20.7±0.4) mb.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452905
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  • 7
    Electronic Resource
    Electronic Resource
    Is coupled cluster singles and doubles (CCSD) more computationally intensive than quadratic configuration interaction (QCISD)? (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 3700-3703 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown that the recently proposed QCI method including all single and double substitutions has essentially the same computational requirements as the more complete CCSD approach. If properly formulated, the CCSD equations contain at most quadratic terms in the excitation amplitudes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455827
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  • 8
    Electronic Resource
    Electronic Resource
    The electric dipole moment of ArH+. Disagreement between theory and experiment (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6486-6490 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The experimentally determined dipole moment of the ArH+ molecular ion (1.4 D, 1.59 D) is in considerable disagreement with previous ab initio calculations (2.2 D). In this work we present further theoretical evidence strongly supporting a dipole moment of 2.1±0.1 D. We have carried out an extensive basis set study and calculated the dipole moment using the coupled cluster analytical energy gradient and polarization propagator techniques. Based on a theoretical determination of the equilibrium rotational g factors for ArH+ and ArD+ we conclude that the extrapolation of the experimentally determined g factors to their equilibrium values is the main source of the discrepancy between theory and experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456314
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  • 9
    Electronic Resource
    Electronic Resource
    An efficient reformulation of the closed-shell coupled cluster single and double excitation (CCSD) equations (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 7382-7387 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The closed-shell CCSD equations are reformulated in order to achieve superior computational efficiency. Using a spin adaptation scheme based on the unitary group approach (UGA), we have obtained a new set of equations that greatly improves our previous formulation. Based on this scheme we have also derived equations for the closed-shell configuration interaction including all single and double excitations (CISD) case. Both methods have been implemented and tested. For a range of test cases the new CCSD method is more efficient than the earlier CCSD method. The new closed-shell CISD procedure is faster than the shape-driven (SD)GUGA algorithm and the new CCSD scheme is less than two times more computation intensive than SDGUGA CISD per iteration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455269
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  • 10
    Electronic Resource
    Electronic Resource
    Analytic evaluation of energy gradients for the single and double excitation coupled cluster (CCSD) wave function: Theory and application (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5361-5373 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory for the analytic evaluation of energy gradients for coupled cluster (CC) wave functions is presented. In particular, explicit expressions for the analytic energy gradient of the CC singles and doubles (CCSD) wave function for a closed-shell restricted Hartree–Fock reference determinant are presented and shown to scale as N6 where N is the one-electron number of atomic basis functions for the molecular system. Thus analytic CCSD gradients are found to be of the same magnitude in computational cost as is the evaluation of analytic gradients for the configuration interaction singles and doubles (CISD) wave function. Applications of this method are presented for the water molecule and the formaldehyde molecule using a double-ζ plus polarization (DZ+P) basis set. The CCSD equilibrium geometries, dipole moments, and, via finite differences of gradients, CCSD harmonic vibrational frequencies and infrared intensities are reported. For H2O these results are compared to analogous CISD, CISDT, CISDTQ, and experimental results, and it is found that the CCSD predictions are most comparable to those of CISDTQ for this particular system. For the case of H2CO, the CCSD results are compared to CISD and experimental predictions. In general, the CCSD results and timings are encouraging.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453655
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