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  • 1
    Electronic Resource
    Electronic Resource
    Restricted open-shell Hartree–Fock-based many-body perturbation theory: Theory and application of energy and gradient calculations (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 6606-6620 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new approach for many-body perturbation theory (MBPT) built upon a restricted open-shell Hartree–Fock (ROHF) reference function is presented. ROHF-MBPT is shown to give much improved results compared to unrestricted Hartree–Fock (UHF) MBPT in cases where there is large spin contamination of the UHF reference function, and to converge much more rapidly to the infinite-order coupled-cluster result. Equations for analytical gradients at the MBPT(2) level are described and implemented. ROHF-MBPT and restricted open-shell Hartree–Fock single- and double-excitation coupled cluster (ROHF-CCSD) applications are presented for several difficult cases. These include the structure and electron affinity of the CN radical; structure, binding energy, and vibrational frequencies of Li3; the structure and vibrational frequencies for the unobserved FCS molecule; and the multiplet structure of the Ni atom.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463664
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  • 2
    Electronic Resource
    Electronic Resource
    A theoretical study of linear carbon cluster monoanions, C−n, and dianions, C2−n (n=2–10) (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 3445-3457 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A large number of carbon cluster monoanions, C−n, have now been detected by negative ion photoelectron spectroscopy. In addition, evidence for carbon cluster dianions, C2−n, as small as C2−7 has been obtained mass spectrometrically. In this research we report results of theoretical calculations of structures and energetics of formation of linear carbon cluster monoanions and dianions containing up to ten carbon atoms. A number of different electronic states have been investigated. Self-consistent field (SCF) theory, many-body perturbation theory, and coupled-cluster theory including triple excitations have been used with basis sets containing polarization and diffuse functions. Considerably larger basis sets have also been used in calculations on some of the smaller species. For the monoanions, the observed electron detachment energies and the even–odd alternation thereof are well reproduced by the calculations. For the dianions, the even numbered species are found to be more easily formed than the odd numbered species, in accord with the intensity pattern observed in the mass spectrometric experiments, and with the availability of partially occupied π orbitals. C2−10 is established to be vertically and adiabatically stable to electron loss, while C2−8 is found to be vertically stable but adiabatically unstable to electron loss. Improved calculations may be sufficient to make C2−8 also stable to adiabatic electron loss. C2−7 and C2−9 are both found to be unstable to vertical electron loss, although both have negative highest occupied molecular orbital (HOMO) eigenvalues and C2−9 is stable to vertical electron loss at the SCF level. The geometry changes resulting from the addition of two electrons are significant, especially for the even numbered clusters. Addition of two electrons to the partially occupied π orbitals of the latter leads to strong single–triple bond alternation, which may be rationalized by noting that the dianions are products of double deprotonation of HC2nH. Such an "accordion'' mechanism may have a role in the ability of carbon clusters to conduct electricity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462980
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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    Harmonic vibrational frequencies and infrared intensities from analytic fourth-order many-body perturbation theory gradients (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 404-413 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently developed fourth-order many-body perturbation theory [MBPT(4)] analytic gradient techniques are applied to a study of the harmonic vibrational frequencies and infrared intensities of prototype small polyatomic molecules. Results are presented for the complete fourth-order model as well as for the SDQ-MBPT(4) scheme, which neglects the contribution of triple excitations. These results are also compared to frequencies and intensities obtained at both higher and lower levels of theory. Differences between the fourth-order results and those obtained with the infinite order coupled-cluster (CC) counterparts of SDQ-MBPT(4) and MBPT(4) [CCSD and CC models including triple excitations, the latter approximated here by CCSDT-1 and the UCC(4) model] are found to be negligible for all bending modes and stretches involving single bonds. However, the infinite order effects included in CC models are important for describing stretching modes of multiple bonds. Therefore, iterative CC calculations are needed to accurately characterize these regions of the potential surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460356
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  • 5
    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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  • 6
    Electronic Resource
    Electronic Resource
    Triple and quadruple excitation contributions to the binding in Be clusters: Calibration calculations on Be3 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8875-8880 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated the contribution of connected triple and quadruple excitations to the binding in Be3 by comparing various coupled-cluster (CC) and truncated configuration-interaction (CI) treatments with multireference CI (MRCI) and full CI (FCI) calculations. The CC method with single and double excitations (CCSD) produces results that differ substantially from more elaborate treatments, but most extensions to CCSD that account approximately for connected triple excitations perform very well. In contrast, good agreement with FCI for Be2 can be achieved only with the highest level CC and MRCI methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459226
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  • 7
    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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  • 8
    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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  • 9
    Electronic Resource
    Electronic Resource
    Linear and cyclic isomers of C4. A theoretical study with coupled-cluster methods and large basis sets (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 8372-8381 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground electronic states of linear and rhombic C4 have been studied by high level ab initio quantum chemical techniques. Geometries, harmonic vibrational frequencies, infrared intensities, and other quantities have been determined using 4s3p2d1f correlation consistent basis sets and coupled-cluster methods including triple excitations. The linear–rhombic isomer energy difference has been investigated with a range of basis sets, including a 5s4p3d2f1g correlation consistent set. The linear–rhombic energy difference is influenced significantly by basis set, presence of triple excitations, and the choice of reference function for the open-shell linear isomer. The effect of basis set variation is complex, but once a reasonable quality of basis set has been achieved, further extensions favor the rhombic isomer. The inclusion of triple excitations also favors the rhombic isomer. The use of a restricted Hartree–Fock reference function for the linear isomer yields higher energies at the coupled-cluster level than if an unrestricted Hartree–Fock reference function is used, thereby again favoring the rhombic isomer. The most complete calculations of this study [coupled-cluster singles and doubles with noniterative triples (CCSD(T)) with a 5s4p3d2f1g basis set] indicate that the rhombic isomer is preferred by about 1 kcal mol−1. The coupled-cluster vibrational frequencies of the linear isomer are all real, in agreement with previous work, indicating that this isomer is not bent in the gas phase. The infrared intensities of linear C4 obtained in this work differ significantly from those obtained previously with smaller basis sets and either self-consistent field theory or second-order perturbation theory. The present calculations give a dissociation energy of C4 of 433 kcal mol−1, which is close to a previous value obtained with the aid of an empirical correction, and implies that several experimental estimates of the heat of formation of C4 are unreliable. Electron detachment energies of linear C4− and electron affinities of C4 are computed with larger basis sets than previously and are in very good agreement with recent anion photoelectron data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463407
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  • 10
    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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