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  • 1
    Electronic Resource
    Electronic Resource
    Structures of some fluorinated benzenes determined by ab initio computation (1982)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 3 (1982), S. 344-353 
    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: The structures of fluorobenzene, 1,3-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3-trifluorobenzene, pentafluorobenzene, and hexafluorobenzene have been determined by the ab initio gradient method with a 4-21 basis set. Approximate corrections have been developed to yield re and r0 structures so that comparison can be made with the available experimental information. The C—F bond distance decreases in a uniform manner with increasing fluorination of the ring, either on adjacent or on meta sites. The ring structure also shows systematic deformation, with a decrease of about 0.01 Å in the C—C bond length adjacent to the point of substitution and an increase of about 2.5˚ in the ring angle at the substitution site in those cases where the requirement of ring closure permits.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540030309
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  • 2
    Electronic Resource
    Electronic Resource
    Methods for parallel computation of SCF NMR chemical shifts by GIAO method: Efficient integral calculation, multi-Fock algorithm, and pseudodiagonalization (1997)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 18 (1997), S. 816-825 
    Details
    ISSN: 0192-8651
    Keywords: parallel computation ; SCF NMR chemical shifts ; GIAO ; pseudodiagonalization ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: We implemented our gauge-including atomic orbital (GIAO) NMR chemical shielding program on a workstation cluster, using the parallel virtual machine (PVM) message-passing system. On a modest number of nodes, we achieved close to linear speedup. This program is characterized by several novel features. It uses the new integral program of Wolinski that calculates integrals in vectorized batches, increases efficiency, and simplifies parallelization. The self-consistent field (SCF) step includes a multi-Fock algorithm, i.e., the simultaneous calculation of several Fock matrices with the same integral set, increasing the efficiency of the direct SCF procedure. The SCF diagonalization step, which is difficult to parallelize, has been replaced by pseudodiagonalization. The latter, widely used in semiempirical programs, becomes important in ab initio type calculations above a certain size, because the ultimate scaling of the diagonalization step is steeper than that of integral computation. Examples of the calculation of the NMR shieldings in large systems at the SCF level are shown. Parallelization of the density functional code is underway. © 1997 by John Wiley & Sons, Inc. J Comput Chem 18: 816-825, 1997
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Characters for symmetric and antisymmetric higher powers of representations: Application to the number of anharmonic force constants in symmetrical molecules (1989)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 10 (1989), S. 935-938 
    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: Explicit formulas are given for the characters of symmetric and antisymmetric powers of an arbitrary representation up to the sixth, and a general method for obtaining the higher ones is described. The results allow, among others, the determination of nonvanishing higher force constants in symmetrical molecules. The benzene molecule, for instance, has 237 nonvanishing cubic and 1890 quartic force constants. Other potential applications are a general method for the symmetry species of vibrational overtones, the determination of the number of independent centrifugal distortion constants, and the symmetry classification of vibrational multiplets.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540100711
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  • 4
    Electronic Resource
    Electronic Resource
    A space-saving modification of Davidson's eigenvector algorithm (1990)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 11 (1990), S. 1164-1168 
    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 modification of Davidson's eigenvalue algorithm, based on the conjugate gradient method, is described. This method needs storage only for a few vectors (five to seven, depending on the implementation), making it practical for very large problems where disk storage is the limiting factor, without the necessity of restarting or discarding some expansion vectors. The convergence characteristics of the modified method are essentially identical with those of the original Davidson method if all expansion vectors are retained in the latter.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540111008
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  • 5
    Electronic Resource
    Electronic Resource
    Comparison of the boys and Pipek-Mezey localizations in the local correlation approach and automatic virtual basis selection (1993)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 14 (1993), S. 736-740 
    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: Our implementation of Pipek-Mezey population localization is described. It is compared with other localization schemes and its use in the framework of the local correlation method is discussed. For such use, this localization is shown to be clearly superior to Boys in the case of physically well-localized systems. Our current algorithm for selection of local virtual spaces is also described. © 1993 John Wiley & Sons, Inc.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540140615
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  • 6
    Electronic Resource
    Electronic Resource
    Ab initio evidence for the stepwise mechanism of the McLafferty rearrangement of the butanal radical cation (1992)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 13 (1992), S. 183-186 
    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: The mechanism of the McLafferty rearrangement of the butanal radical cation to ethylene and vinyl alcohol cation is found, by ab initio calculations, to be stepwise. The results of a previous ab initio study are inconclusive because of symmetry restriction in their geometry optimization.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540130210
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  • 7
    Electronic Resource
    Electronic Resource
    Density functional based vibrational study of conformational isomers: Molecular rearrangement of benzofuroxan (1997)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 18 (1997), S. 489-500 
    Details
    ISSN: 0192-8651
    Keywords: benzofuroxan ; ortho-dinitrosobenzene ; nitrosobenzene ; vibrational spectra ; tautomerism ; density functional theory (DFT) ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The molecular rearrangement of benzofuroxan was studied by comparing calculated and experimental IR spectra, the latter taken before and during the reaction. All calculations were performed at the B3-LYP/6-31G(d) density functional level with a further refinement of the computed force constants done by applying the scaled quantum mechanical force field (SQM) technique. Complete assignments for the IR spectra of benzofuroxan and nitrosobenzene are given. The agreement between computed and experimental spectra is excellent, but in benzofuroxan these spectra are very different from previously calculated data. The conformation of the ortho-dinitrosobenzene intermediate of this tautomeric reaction was identified by modeling a composite IR spectrum of four possible components. It shows good agreement with an experimental spectrum that was obtained after photolysing benzofuroxan in Xe matrix. Knowing the conformation of the intermediate provides insight into the reaction mechanism and allows inferences for the thermal reaction, which could not be clarified conclusively by energetic considerations only. © 1997 by John Wiley & Sons, Inc. J Comput Chem 18: 489-500, 1997
    Additional Material: 5 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    Predicting the vibrational spectra of some simple fluorocarbons by direct scaling of primitive valence force constants (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 19 (1998), S. 1187-1204 
    Details
    ISSN: 0192-8651
    Keywords: vibrational spectra ; SQM force fields ; fluorocarbons ; density functional theory ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Using the scaled quantum-mechanical (SQM) force field approach with direct scaling of individual primitive force constants, we derive optimal scaling factors by a least-squares fit to the experimentally observed fundamentals of some selected “simple” fluorocarbons. We use the derived scaling factors to predict the vibrational spectra of all possible fluoromethanes, fluoroethylenes, fluoroethanes, and monofluoropropenes, proposing a reassignment of some experimental fundamentals. Two separate sets of scaling factors are derived for both traditional Hartree-Fock (HF) calculations and density functional theory (DFT) calculations using the hybrid three-parameter B3-PW91 density functional. With the split-valence 6-31G(d) basis set, our scaling procedure gives an average error of less than 9 cm-1 in the scaled frequencies with the B3-PW91 functional. The average percentage error is around 1%. The HF results are not as good - the average error is 12.6 cm-1 - showing that hybrid density functional SQM schemes are better for predicting vibrational spectra than basic Hartree-Fock.   © 1998 John Wiley & Sons, Inc.   J Comput Chem 19: 1187-1204, 1998
    Additional Material: 2 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Integral transformation with low-order scaling for large local second-order Møller-Plesset calculations (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 19 (1998), S. 1241-1254 
    Details
    ISSN: 0192-8651
    Keywords: integral transformation ; low-order scaling ; second-order Møller-Plesset calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: An algorithm is presented for the four-index transformation of electron repulsion integrals to a localized molecular orbital (MO) basis. Unlike in most programs, the first two indices are transformed in a single step. This and the localization of the orbitals allows the efficient neglect of small contributions at several points in the algorithm, leading to significant time savings. Thresholds are applied to the following quantities: distant orbital pairs, the virtual space before and after the orthogonalizing projection to the occupied space, and small contributions in the transformation. A series of calculations on medium-sized molecules has been used to determine appropriate thresholds that keep the truncation errors small (below 0.01% of the correlation energy in most cases). Benchmarks for local second-order Møller-Plesset perturbation theory (MP2; i.e., MP2 with a localized MO basis in the occupied subspace) are presented for several large molecules with no symmetry, up to 975 contracted basis functions, and 60 atoms. These are among the largest MP2 calculations performed on a single processor. The computational time (with constant basis set) scales with a somewhat lower than cubic power of the molecular size, and the memory demand is moderate even for large molecules, making calculations that require a supercomputer for the traditional MP2 feasible on workstations.   © 1998 John Wiley & Sons, Inc.   J Comput Chem 19: 1241-1254, 1998
    Additional Material: 3 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    Ab initio geometry optimization for large molecules (1997)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 18 (1997), S. 1473-1483 
    Details
    ISSN: 0192-8651
    Keywords: geometry optimization ; DIIS optimization algorithm ; Hessian ; natural internal coordinates ; rational function ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Various geometry optimization techniques are systematically investigated. The rational function (RF) and direct inversion in the iterative subspace (DIIS) methods are compared and optimized for the purpose of geometry optimization. Various step restriction and line search procedures are tested. The model Hessian recently proposed by Lindh et al. has been used in conjunction with different Hessian update procedures. Optimization for over 30 molecules have been performed in Z-matrix coordinates, local normal coordinates, and curvilinear natural internal coordinates, using the same approximations for the Hessian in all cases. The most effective and stable procedure for optimization of equilibrium structures was found to be the DIIS minimization in natural internal coordinates using the BFGS update of the model Hessian. Our method shows faster overall convergence than all previously published methods for the same test suite of molecules.   © 1997 John Wiley & Sons, Inc.   J Comput Chem 18: 1473-1483, 1997
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