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  • Physical Chemistry  (2)
  • Theoretical, Physical and Computational Chemistry  (2)
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  • 1
    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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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Chemical Kinetics 25 (1993), S. 285-303 
    ISSN: 0538-8066
    Keywords: Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A kinetic mechanism for the chain decomposition of formaldehyde consistent with recent theoretical and experimental results is presented. This includes new calculations and measurements of the rate constant for the abstraction reaction \documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm CH}_{\rm 2} {\rm O + H} \to {\rm HCO + H}_{\rm 2} $$\end{document} The calculation uses a multi-reference configuration interaction wavefunction to construct the potential energy surface which is used in a tunneling-corrected TST calculation of the rate constant. The rate constant for the bond fission \documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm CH}_{\rm 2} {\rm O + M} \to {\rm HCO + H + M} $$\end{document} at high temperatures was determined by an RRKM extrapolation of direct low temperature measurements. This mechanism has been successfully tested against laser-schlieren measurements covering the temperature range 2200-3200 K. These measurements are insensitive to all but the above two reactions and they confirm the large, non-Arrhenius rate for the abstraction reaction derived here from theory. Modeling of previous experiments using IR emission, ARAS, and CO laser absorption with this mechanism is quite satisfactory. The branching ratio of the rate of the faster molecular dissociation (CH2O + (M) → CO + H2 + (M)), to that of the bond fission reaction, was estimated to be no more than 2 or 3 over 2000 to 3000 K. Such a ratio is consistent with one recent theoretical estimate and most of the experimental observations. © 1993 John Wiley & Sons, Inc.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0192-8651
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Several parallel algorithms for Fock matrix construction are described. The algorithms calculate only the unique integrals, distribute the Fock and density matrices over the processors of a massively parallel computer, use blocking techniques to construct the distributed data structures, and use clustering techniques on each processor to maximize data reuse. Algorithms based on both square and row-blocked distributions of the Fock and density matrices are described and evaluated. Variants of the algorithms are discussed that use either triple-sort or canonical ordering of integrals, and dynamic or static task clustering schemes. The algorithms are shown to adapt to screening, with communication volume scaling down with computation costs. Modeling techniques are used to characterize algorithm performance. Given the characteristics of existing massively parallel computers, all the algorithms are shown to be highly efficient for problems of moderate size. The algorithms using the row-blocked data distribution are the most efficient. © 1996 by John Wiley & Sons, Inc.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 0538-8066
    Keywords: Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reactants, products, and saddle point for the reaction H2 + CN → H + HCN have been studied by ab initio calculations. The computed structures, frequencies, and energetics are compared directly to available measurements and, indirectly, to experimental rateconstants. The theoretical rate constants used in the comparison are calculated with conventional transition state theory. By reduction of the computed reaction barrier to 4.1 kcal mol,-1 good agreement with experimental rate constants is obtained over a 3250-K temperature range. This computed rate constant is well represented by the form 4.9 × 10-18 T2.45 e-1, 126/T over the temperature range of 250 K-3500 K. Substantial reaction rate curvature is found due to low-frequency bending modes at the saddle point. The results for this reaction are compared to other abstraction reactions involving H atom transfer to identify correlations between reaction exothermicity and both abstraction barriers and reaction rate curvature.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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