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  • 1
    Electronic Resource
    Electronic Resource
    Stability and properties of C4 isomers (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 3612-3617 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The relative electronic energies of the 1Ag rhombus and the 3Σ−g linear isomers of C4 have been computed using a 5s3p1d basis at various levels of coupled-cluster and many-body perturbation theory. At the highest level performed, CCSD+T(CCSD), the two isomers are essentially isoenergetic. Themodynamic results indicate, however, that the linear isomer will be significantly more abundant in the plasma in which C4 is formed. Vibrational frequencies are presented and compared with experimental results; some of the prior assignments are questioned. The isotropic ESR hyperfine splitting parameters for the linear triplet state have been computed and are also compared with experimental results. Finally, the ionization potential and electron affinity for both structures are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454881
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  • 2
    Electronic Resource
    Electronic Resource
    Orbital-invariant second-order many-body perturbation theory on parallel computers: An approach for large molecules (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 9582-9589 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equations for the second-order many-body perturbation theory [MBPT(2)] energy are derived in an orbital-invariant representation, analogous to that obtained with the method of self-consistent electron pairs of Meyer. This formulation is well suited to take advantage of the localized nature of interactions in large chemical systems in order to reduce the computational effort required to study them. This formulation of the MBPT(2) method also lends itself to implementation on parallel computers. We describe a scalable implementation in which the key data are distributed across the parallel computer rather than being replicated. Portability to both shared- and distributed-memory computer architectures is provided through the use of a subroutine library implementing a "global array'' programming model. We demonstrate that this approach is scalable even for relatively small chemical systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468774
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  • 3
    Electronic Resource
    Electronic Resource
    The structure and binding energy of K+–ether complexes: A comparison of MP2, RI-MP2, and density functional methods (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 1940-1950 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structures and binding energies of several cation:ether complexes (K+:dimethyl ether, K+:dimethoxyethane, K+:12-crown-4 and K+:18-crown-6) were determined with second and fourth order perturbation theory using correlation consistent basis sets. Several of these are the largest correlated calculations yet attempted on crown ethers. The observed systematic convergence to the complete basis set limit provides a standard by which the accuracy of previous studies can be measured and facilitates the calibration of density functional methods. Recent Fouier transform ion cyclotron resonance experiments predicted K+:18-crown-6 binding energies which were significantly smaller than ab initio calculations. None of the potential sources of error examined in the present study were large enough to explain this difference. Although the 6-31+G* basis set used in an earlier theoretical study was smaller than the smallest of the correlation consistent basis sets, with suitable correction for basis set superposition error, it appears capable of yielding binding energies within several kcal/mol of the basis set limit. Perturbation theory calculations exploiting the "resolution of the identity'' approximation were found to faithfully reproduce binding energies and conformational differences. Although the cation–ether interaction is dominated by classical electrostatics, the accuracy of density functional techniques was found to be quite sensitive to the choice of functionals. The local density SVWN procedure performed well for binding energies and conformational differences, while underestimating K+O distances by up to 0.08 A(ring). The gradient-corrected Becke–Lee–Yang–Parr functional underestimated the K+:12c4 binding energy by 4–7 kcal/mol or 15%. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472082
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  • 4
    Electronic Resource
    Electronic Resource
    A theoretical study of the structure, bonding, and vibrational frequency shifts of the H2–HF complex (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 5120-5127 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The complex formed from hydrogen fluoride and the hydrogen molecule has been investigated theoretically in two ways. Large basis, well-correlated ab initio electronic structure calculations have been used to map out regions of the potential energy surface and an electrical interaction model has been used to find the classical intermolecular interaction effects. From the ab initio potential surface, the fundamental vibrational transition frequencies of hydrogen and hydrogen fluoride are predicted to be red shifted by 20 and 15 cm−1, respectively. The Liu and Dykstra theory of vibrational frequency shifts that uses the intermolecular electrical interaction yields shifts of 31 and 19 cm−1, respectively. The equilibrium structure of the molecule is T shaped, a feature that is determined by electrical interaction, and the well depth is around 300 cm−1. A significant fraction of the well-depth results from electron correlation effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451705
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  • 5
    Electronic Resource
    Electronic Resource
    Fitting basis sets for the RI-MP2 approximate second-order many-body perturbation theory method (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 1593-1600 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The "resolution of the identity" integral approximation applied to second-order many-body perturbation theory, or RI-MP2, method offers improved computational performance compared to traditional (exact) second-order perturbation theory calculations, but introduces a new auxiliary or "fitting basis set" into the method. We develop fitting basis sets for use with the correlation consistent cc-pVDZ and cc-pVTZ atomic orbital basis sets for the atoms H-Ne. These fitting sets are designed to reproduce exact second-order results for a set of 32 test cases, including a variety of reaction energies, weak interactions, and electrostatic properties, to better than 1% error averaged across all tests and less than 2% error in any individual case. Although the RI-MP2 method is primarily targeted to large-scale calculations, it offers substantial performance improvements even for the small molecules used in these test cases. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476732
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  • 6
    Electronic Resource
    Electronic Resource
    Localized Hartree product orbitals in correlated studies of molecules (1994)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 49 (1994), S. 559-573 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Use of orthogonalized Hartree product (OHP) orbitals as the reference orbitals in coupled-cluster (cc) calculations is presented. Since such orbitals are determined without exchange, they provide as “classical” a description as possible. The OHP orbitals were generated by implementing Harris's formulation of the original Hartree method. Some computational considerations of the formulation are discussed. A critical evaluation of the OHP method as an orbital localization scheme is presented. The OHP orbitals were used as the reference in CCD and CCSD calculations and compared with corresponding Hartree-Fock (HF) reference CC results. The average variation of localized Hartree product (LHP) reference CCSD energy from that of the HF reference is 0.83 kcal/mol, whereas for CCD, the average variation is 234 kcal/mol, indicating the importance of single-excitation effects in CC calculations with non-HF references. © 1994 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560490502
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  • 7
    Electronic Resource
    Electronic Resource
    Concise, open-ended implementation of Rys polynomial evaluation of two-Electron integrals (1990)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 11 (1990), S. 972-977 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A quadrature-point-driven implementation of the standard Rys polynomial method for computing two-electron repulsion integrals of gaussian basis functions has been found to be both concise and openended with respect to the angular momentum of the gaussian functions (i.e., s,p,d,f,g,…). These are important features in certain applications, such as molecular properties and property gradients.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540110809
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  • 8
    Electronic Resource
    Electronic Resource
    Toward high-performance computational chemistry: II. A scalable self-consistent field program (1996)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 17 (1996), S. 124-132 
    Details
    ISSN: 0192-8651
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: We discuss issues in developing scalable parallel algorithms and focus on the distribution, as opposed to the replication, of key data structures. Replication of large data structures limits the maximum calculation size by imposing a low ratio of processors to memory. Only applications which distribute both data and computation across processors are truly scalable. The use of shared data structures that may be independently accessed by each process even in a distributed memory environment greatly simplifies development and provides a significant performance enhancement. We describe tools we have developed to support this programming paradigm. These tools are used to develop a highly efficient and scalable algorithm to perform self-consistent field calculations on molecular systems. A simple and classical strip-mining algorithm suffices to achieve an efficient and scalable Fock matrix construction in which all matrices are fully distributed. By strip mining over atoms, we also exploit all available sparsity and pave the way to adopting more sophisticated methods for summation of the Coulomb and exchange interactions. © 1996 by John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
  • 9
    Electronic Resource
    Electronic Resource
    An empirically adjusted Newton-Raphson algorithm for finding local minima on molecular potential energy surfaces (1990)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 11 (1990), S. 58-63 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A simple extension of the Newton-Raphson method is proposed that approximately accounts for anharmonicity in bond-stretching coordinates. By modeling each bonded distance in a polyatomic molecule as a Morse oscillator with no anharmonic stretch-stretch or stretch-bend coupling, a multiplicative correction factor to the Newton-Raphson step is derived. Representative examples suggest that the rate of convergence of the proposed scheme is typically faster than that of the standard Newton-Raphson method.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540110106
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