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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
    Ab initio self-consistent-field molecular orbital calculations on defects associated with radiation damage in alpha quartz (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4215-4224 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A series of ab initio self-consistent-field molecular orbital (SCF-MO) calculations, on defect centers involved in radiation damage of α quartz, is reported. Calculations were performed on a 21 atom cluster (Si5O12−16), using a 3–21 G basis set. The long-range Coulomb interactions of α quartz were included, by surrounding the 21 atom cluster with 956 point charges, so that on geometry optimization, the characteristic features of α quartz were maintained. Various defect centers containing Al3+ substitutional ions, and their compensating electron-hole and proton defects were studied. The results obtained for the [AlO4]0 center are in close agreement with electron spin resonance (ESR) data. Calculations were also performed on the oxygen vacancy and related defects, such as the E'1 and E4 defects, where qualitative agreement with ESR results was obtained. An assessment is made, of the effect of including long-range Coulomb interactions, by comparison with other SCF-MO cluster models of these systems, which do not include these electrostatic effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460776
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  • 4
    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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  • 5
    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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  • 6
    Electronic Resource
    Electronic Resource
    The argon hydrogen–fluoride differential scattering cross section (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 2182-2191 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The total differential cross section for Ar–HF was measured at a collision energy of 1637 K (141 meV). Although diffraction oscillations were not resolved, a broad primary rainbow peak was observed. Scattering calculations were carried out for the Ar–HF interaction potentials developed by Douketis et al. and Hutson and Howard. The spherical potential and infinite order sudden approximations do not give a good description of the total differential scattering. The final-l labeled coupled states approximation, on the other hand, is in good agreement with the more accurate close coupled approximation. Neither potential predicts cross sections which are in good agreement with the measured scattering intensities. Contributions to the total scattering from elastic and inelastic processes were investigated using the coupled states approximation. The contributions to the total scattering from different initial HF rotational states were also studied. The total scattering for j initial=0 differs significantly from that for other values of j initial. It is therefore important to know the distribution of rotational states in the HF beam if one is to compare calculated total differential cross sections with measured ones. The largest inelastic cross section is for the j=0 to j'=1 transition. Collisions in which the Ar atom interacts strongly with the anisotropic potential well, rather than near head-on collisions off the repulsive wall of the potential, are responsible for the large j=0 to j'=1 cross section. The results of accurate SCF calculations for Ar–HF are also reported in this paper.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456013
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  • 7
    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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  • 8
    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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  • 9
    Electronic Resource
    Electronic Resource
    Diffusion Monte Carlo simulations of hydrogen fluoride dimers (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 603-616 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The diffusion Monte Carlo method is used to simulate vibrational states for HF and DF monomers and dimers. Ground state energies, dissociation energies, and vibrational motions are discussed on the basis of the simulations. Vibrational frequencies for the dimer are calculated using a mean field, local mode analysis, local mode variational calculations, and direct excited state diffusion Monte Carlo simulations. A tunneling splitting calculation is done using a fixed node simulation. Three different potential surfaces are used for the calculations and results are compared with the available experimental data. One of the surfaces is modified on the basis of the calculations to give improved agreement with experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458410
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  • 10
    Electronic Resource
    Electronic Resource
    The infrared spectrum and structure of hydrogen fluoride clusters and the liquid: Semiclassical and classical studies (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 1810-1821 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Computer simulation methods are used to calculate the vibrational spectra of hydrogen fluoride clusters, and the liquid, in the intramolecular stretching region. A semiclassical approach, in which Monte Carlo and molecular dynamics simulations are analyzed using quantum vibrational calculations, is a considerable improvement over dipole correlation function methods. Comparisons with experimental data show that induced dipole interactions play an important role in determining spectral shifts. A simple Drude oscillator model of polarization allows the spectral shifts to be obtained correctly, but at the expense of overestimating structural correlations in the liquid.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462082
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