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  • 1
    Electronic Resource
    Electronic Resource
    Theoretical Characterization of the Ground States of HNS and NSH (1995)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 99 (1995), S. 5331-5335 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100015a015
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  • 2
    Electronic Resource
    Electronic Resource
    Analytic energy gradients with frozen molecular orbitals in coupled-cluster and many-body perturbation theory methods: Systematic study of the magnitude and trends of the effects of frozen molecular orbitals (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3853-3863 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Analytic coupled-cluster (CC) and many-body perturbation theory (MBPT) energy gradient methods with restricted Hartree–Fock (RHF), unrestricted Hartree–Fock (UHF), restricted open-shell Hartree–Fock (ROHF), and quasi-RHF(QRHF) reference functions are extended to permit dopping core and excited orbitals. By using the canonical property of the semicanonical ROHF orbitals and the RHF orbitals from which the QRHF reference function is constructed, it is shown that a general procedure can be established not only for RHF and UHF, but also for ROHF and QRHF reference functions. The basic theory and implementation are reported. To provide a systematic study of the trends and magnitudes of the effects of dropped molecular orbitals (MOs) on the structures, harmonic frequencies, and ir intensities, we study HCN, C2H2, CO2, HO2, and C2H4 at increasing levels of correlation and basis sets. The effects of the dropped MOs with the largest basis sets are about 0.003 Å and 0.1° in structures and about 1% on harmonic frequencies and ir intensities. The magnitude and the direction of the drop-MO effect tend to be almost constant from MBPT(2) to CCSD(T) methods. The two isomers of S3 are studied by the drop-MO-method, yielding very accurate results. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474744
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  • 3
    Electronic Resource
    Electronic Resource
    Coupled-cluster calculations of structure and vibrational frequencies of ozone: Are triple excitations enough? (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2511-2514 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Coupled-cluster calculations with full inclusion of singles, doubles, and triples (CCSDT) with a double-zeta plus polarization and correlation-consistent polarized valence triple-zeta basis sets have been used to calculate the structure and harmonic vibrational frequencies of the ozone molecule. These results have been compared with those of more approximate CC methods, and the effects of the different terms in the triple excitation equation are analyzed. The effect of basis set extension on the CCSDT results has been estimated. In the limit of a large basis set, it appears that the complete CCSDT method will give smaller bond lengths than experiment and an asymmetric stretching frequency about 50 cm−1 above the experimental value. That is, it would appear that connected quadruple excitations are needed for quantitative calculations of the structure and frequencies of ozone. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475634
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  • 4
    Electronic Resource
    Electronic Resource
    The inclusion of connected triple excitations in the equation-of-motion coupled-cluster method (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3073-3078 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the implementation of connected triple excitations in the equation-of-motion (EOM) coupled-cluster (CC) method for excitation energies for the first time. The reference state is described by the complete CC singles, doubles, and triples (CCSDT) method. Excited states are generated from the reference state wave function by the action of a linear excitation operator including single, double, and triple excitations. The excited state wave functions and energies are obtained by diagonalizing the effective Hamiltonian e−THeT, where T is the cluster operator for the reference state, in the space of singly, doubly, and triply excited determinants. Comparison is made with full configuration interaction excitation energies for several examples (CH+, Be, SiH2, and CH2). These show that EOM-CCSDT is able to describe states which are doubly excited relative to the reference state, as well as singly excited states. Calculations of several excitation energies of BH using an extended basis set are also reported, and show good agreement with experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467620
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  • 5
    Electronic Resource
    Electronic Resource
    A theoretical study of hyperfine coupling constants (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 1425-1434 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Isotropic hyperfine coupling constants of first-row atoms from B–F and the BH2 radical are calculated analytically from the coupled-cluster (CC) relaxed density with a variety of extended basis sets. We employ both restricted and unrestricted Hartree–Fock reference functions, with the CC singles and doubles (CCSD), CCSD with noniterative triples [CCSD+T(CCSD) and CCSD(T)] methods. The latter provide excellent agreement with experiment. We also consider the role of orbital relaxation and atomic basis functions in accurate predictions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466620
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  • 6
    Electronic Resource
    Electronic Resource
    Coupled-cluster calculations on the C2 molecule and the C+2 and C−2 molecular ions (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 6073-6084 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Coupled-cluster methods including effects of triple excitations have been used with large basis sets to study several electronic states of C2, C−2, and C+2. re, ωe, De, and Te have been computed for each state considered. For those states for which experimental data are available, the errors are typically less than 0.002 A(ring), 20 cm−1, and 0.1 eV for re, ωe, and De, respectively. These errors are comparable with the best previous results on C2 and significantly smaller than those for previous calculations on C−2 and C+2. For Te the results are also satisfactory and comparable with previous work, but some difficulties arise when one or both of the states have considerable multireference character. For two states of C+2 not experimentally characterized, the present data should be sufficiently accurate to be of some assistance to experimental studies.The electron affinity of the ground state of C2 and the ionization potentials of the ground and first excited states of C2 have been computed, and particular attention has been given to correlation errors in these quantities. It has been shown that triple excitations significantly decrease the electron affinity. By neglecting or taking insufficient account of triple excitations, some previous calculations were biased towards the anion and obtained fortuitously good agreement with experiment. The correlation error of the present calculations is such that the calculations are slightly biased towards the neutral molecule, but the error is only 0.2 eV. For the ionization potentials, too, triple excitations play an important role. Reasonable agreement with experiment has been achieved by the present calculations. Overall, the present study: (1) underscores the importance of triple excitations; (2) shows the success of single-reference coupled-cluster theory in calculating properties of a single state, even when that state has considerable multireference character; and (3) highlights the care which must be taken when comparing different electronic states with these methods when one or both states have considerable multireference character.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462649
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  • 7
    Electronic Resource
    Electronic Resource
    A coupled-cluster study of inversion symmetry breaking in the F+2 molecular ion (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 6652-6657 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Several coupled-cluster methods have been used to calculate equilibrium properties (re, ωe, and De ) of the ground state (2Πg) of the F+2 molecular ion. Two unrestricted Hartree–Fock reference determinants have been used. The first was made up of inversion symmetry constrained D2h orbitals, the second of inversion symmetry broken C2v orbitals. The results of the coupled-cluster calculations are rather insensitive to the choice of reference determinant, in contrast to what is observed for finite-order perturbation theory. It follows that for certain symmetry breaking problems, single reference coupled-cluster methods are sufficiently powerful to overcome a symmetry broken reference function, thus in principle obviating the need for either a multireference starting point or a symmetry constrained single reference starting point. Some extended basis set coupled-cluster calculations of equilibrium properties of F+2 and F2 were performed. Very good agreement with experiment was obtained for F2, suggesting that the results for F+2 are also reliable.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461535
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  • 8
    Electronic Resource
    Electronic Resource
    A direct product decomposition approach for symmetry exploitation in many-body methods. I. Energy calculations (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4334-4345 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An analysis of the matrix contractions involved in many-body perturbation theory and coupled-cluster calculations leads to a convenient strategy for exploiting point group symmetry, by which the number of floating point operations can be reduced by as much as a factor of h2, where h is the order of the molecular point group. Contrary to a statement in the literature, the significant reduction in computation time realized in coupled-cluster calculations which exploit symmetry is not due to nonlinearities in the equations. Rather, the savings of the fully vectorizable direct product decomposition (DPD) method outlined here is associated with individual (linear) contractions, and is therefore applicable to both linear and nonlinear coupled-cluster models, as well as many body perturbation theory. In addition to the large reduction in floating point operations made possible by exploiting symmetry, core memory requirements are also reduced by a factor of ≈h2. Implementation of the method for both open and closed shells is reported. Computer timings and hardware requirements are given for several representative chemical systems. Finally,the DPD method is applied to the calculation of the equilibrium geometry, totally symmetric harmonic force field and vertical ionization potentials of the cubane molecule at the coupled-cluster singles and doubles (CCSD) level.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460620
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  • 9
    Electronic Resource
    Electronic Resource
    A coupled-cluster study of the ground state of C+3 (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4320-4327 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report calculations of the structure and harmonic frequencies of the 2B2 state of C+3 and the energy of this state relative to the linear geometry (2Σ+u) using self-consistent-field (SCF) and coupled-cluster (CC) methods, including the full coupled-cluster single, double, and triple excitation (CCSDT) model. The calculations on 2B2 C+3 are the most complete treatment to date and may be of assistance in further experimental detection of this species. The calculations of the 2Σ+u–2B2 energy difference support a bent structure. These calculations also show that, compared with full CCSDT and configuration interaction single, double, triple, and quadruple excitation (CISDTQ) results, CC methods which only approximately include effects of connected triple excitations seem for this example to give misleadingly small energy differences. A recent prediction by such approximate methods that C+3 may be quasilinear is therefore questionable. In the course of this work, certain practical difficulties in the SCF description of 2B2 C+3 were encountered, namely symmetry breaking of the restricted open-shell Hartree–Fock (ROHF) wave function and the existence of two distinct 2B2 unrestricted Hartree–Fock (UHF) solutions. We show that these can be alleviated by using our quasirestricted Hartree–Fock CC approach (QRHF-CC). Coupled-cluster single and double excitation (CCSD) calculations based on a QRHF reference function consisting of neutral molecule orbitals are able to provide all frequencies, unlike those based on a ROHF reference function. This work shows that QRHF-CC calculations offer a convenient single reference solution to certain problems involving symmetry breaking or other difficulties which traditionally have been solved by multireference methods. This quality of the QRHF-CC approach appears not to have been recognized previously.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460618
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  • 10
    Electronic Resource
    Electronic Resource
    The coupled-cluster single, double, and triple excitation model for open-shell single reference functions (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6104-6105 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The CCSDT model for general single determinant reference functions for open and closed-shell electronic states has been implemented for the first time and has been used to compute the electron affinity of the F atom, the CH2, 3B1–1A1 energy difference, and the ionization potentials of 1A1 CH2. The results compare very well with FCI and are markedly superior to those of simpler coupled-cluster methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459002
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