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  • Theoretical, Physical and Computational Chemistry  (143)
  • Wiley-Blackwell  (143)
  • American Chemical Society
  • American Institute of Physics (AIP)
  • 2000-2004
  • 1995-1999  (143)
  • 1955-1959
  • 1950-1954
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  • Wiley-Blackwell  (143)
  • American Chemical Society
  • American Institute of Physics (AIP)
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  • 2000-2004
  • 1995-1999  (143)
  • 1955-1959
  • 1950-1954
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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.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1195-1208 
    ISSN: 0020-7608
    Keywords: GABAB analogs ; pharmacophoric pattern ; molecular similarity ; quantum chemical calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In order to determine the structural requirements that are important for GABAB binding affinity, a quantum-chemical-based conformational study has been performed, followed by a similarity analysis which includes 12 GABAB analogs. Due to the flexibility of the structures, a semigrid GABAB analog [2RS-(5,5-dimethyl) morpholinyl-acetic acid] has been used as a template for the amonium moiety in order to help to identify the active conformation. Both in vacuo, and solvent-simulated calculations, for the physiological media modeled as water molecules, have been compared, for this analog, at ab initio (G94, 6-31+G(d,p)) and semiempirical (PM3) levels, respectively. On the basis of this comparison, the results of in vacuo PM3 calculations have been chosen for the similarity analysis. We have included, in the calculations, a group of molecules heterogeneous enough to become representative of the different families that can bind to the GABAB receptor site. Following their comparison we report the leading characteristics that can be related to their binding capability and define a pharmacophoric pattern for GABAB analogs. The latter is compared with the one previously found for the binding affinity at the GABAA receptor site.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1195-1208, 1998
    Additional Material: 9 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 63 (1997), S. 861-874 
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Direct ab initio dynamics calculations based on a canonical variational transition-state theory with several multidimensional semiclassical tunneling approximations were carried out to obtain rate constants for the water-assisted tautomerization of formamide. The accuracy of the density functionals, namely, B-LYP, B3-LYP, and BH&H-LYP, were examined. We found that the BH&H-LYP method yields the most accurate transition-state properties when comparing it to ab initio MP2 and QCISD results, whereas B-LYP and B3-LYP methods predict barrier heights too low. Reaction path information was calculated at both the MP2 and nonlocal hybrid BH&H-LYP levels using the 6-31G(d,p) basis set. At the BH&H-LYP level, we found that the zero-point energy motion lowers the barrier to tautomerization in the formamide-water complex by 3.6 kcal/mol. When tunneling is considered, the activation energy at the BH&H-LYP level at 300 K is 17.1 kcal/mol. This is 3.4 kcal/mol below the zero-point-corrected barrier and 7.0 kcal/mol below the classical barrier. Excellent agreement between BH&H-LYP and MP2 rate constants further supports the use of BH&H-LYP for rate calculations of large systems. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 861-874, 1997
    Additional Material: 8 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 299-304 
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In Article I, we showed that the freeon-exchange theory of magnetic coupling is a viable alternative to the widely used spin-exchange theory. In Article II, we are concerned with the symmetric-bridge, molecular magnet, AXB, where A and B are magnetic atoms and X is the bridge. The symmetric-bridge molecular magnet is of particular interest because the major features of its spectrum and its coupling constants are obtained by means of the two-electron, two-site freeon theory. We exhibit one example of ferromagnetic coupling, J〉0 and one of antiferromagnetic coupling, J〈0.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 299-304, 1997
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 69 (1998), S. 117-128 
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The requirements for H2-antagonist activity so far identified for most of the known antagonists of histamine are the presence of a heterocyclic ring containing a basic center linked via a methylene chain to a substituted guanidine or thiourea polar side chain. Metiamide is a potent H2 antagonist (pA2=6.06). We have used the ab initio Hartree-Fock (HF) method in order to study the conformational properties of the N3(SINGLE BOND)H tautomers of metiamide molecule and histamine monocation. Three basis set (the 3-21G*, 6-31G**, and 6-31+G**) were used, the results compared, and the geometric parameters fully optimized. Our results indicate the preference of metiamide for a folded conformation with an intramolecular hydrogen bonding between the imidazole ring and one of the NH groups. The optimized geometrical parameters and charge distributions of both molecules, using the Mulliken, and natural bond order (NBO) analysis, are given and discussed.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 117-128, 1998
    Additional Material: 5 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 341-350 
    ISSN: 0020-7608
    Keywords: alumina ; periodic SCF ; surface potential ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic structure of the (001) and (110) α-alumina surfaces was determined by periodic Hartree-Fock method in statically relaxed geometries. Both (001) and (110) surfaces generate specific surface states into the energy gap. Significant influence of charge-transfer effect on the surface-adsorbate potential is possible in both cases studied. The inclusion of the exponential part to the standard potential forms was suggested for the description of the interaction potential in such cases.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 341-350, 1998
    Additional Material: 9 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1127-1136 
    ISSN: 0020-7608
    Keywords: pharmacophoric pattern ; antiepileptic activity ; conformational analysis ; N-valproyl glycine ; N-valproyl glycinamide ; valpromide ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two closely related N-substituted valpromide derivatives: N-valproyl glycinamide and N-valproyl glycine are comparatively analyzed, the first of which is antiepileptic active whereas the second is not. The study is based on a conformational analysis using an AM1 Hamiltonian that not only search for the lower energy structures of each derivative but also for the energy involved in their mutual interconversion. Open structures have been compared with cyclic ones, the latter including those stabilized by either inter or intra molecular hydrogen bonds (dimers and monomers, respectively). H-bond formation has been also evaluated by means of ab initio G94(6-31+G(d,p)) calculations for a smaller system (N-formylglycine/glycinamide) modeling both vacuum and solvent conditions. The conformational and electronic characteristics of the open and cyclic monomers, as well as of the dimer N-valproyl glycinamide and N-valproyl glycine structures are discussed. On the basis of the results of their comparative analysis, we have redefined the pharmacophore previously proposed for N-substituted valpromides [Tasso, Bruno-Blanch, Estiu, Int. J. Quant. Chem. 65(6), 1107 (1997)], relaxing some of the associated requirements. The corrected model requires one carbon atom or any bioisosteric substituent in an anticlinal conformation relative to the aminic nitrogen of the amide moiety, in addition to one hydrogen atom that should be antiperiplanar to the carbonyl oxygen. This model offers an explanation to the different response of N-valproyl glycinamide and N-valproyl glycine against convulsion, which is based on conformational restrictions.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1127-1136, 1998
    Additional Material: 6 Ill.
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  • 8
    ISSN: 0894-3230
    Keywords: N-(aminoalkyl)-9-phenanthrenecarboxamides ; molecular structures ; folded conformations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---The molecular structures of three tertiary N-(aminoalkyl)-9-phenanthrenecarboxamides were investigated in solution and the solid state by means of 1H NMR spectroscopy and x-ray crystallography. The tertiary amides exist as a mixture of E and Z isomers in solution and the aminoalkyl groups exist as a mixture extended and folded conformers. A crystalline N-(aminoethyl)amide was obtained as the pure Z isomer in which the phenanthrene and amide planes are nearly perpendicular and the aminoethyl group is folded over the less hindered face of the amide group. Rotation about the Ar-C(O) bond is slow in resolution rendering these molecules chiral on the NMR time-scale. As a consequence, the α-methylene protons display large diasterotopic splittings when the aminoalkyl group is syn to the amide carbonyl. Folded conformations place the Z and E aminoalkyl groups in the deshielding and shielding regions, respectively, of the phenanthrene rings, resulting in large differences in chemical shifts. © 1997 John Wiley & Sons, Ltd.
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  • 9
    ISSN: 0894-3230
    Keywords: Electron transfer properties ; substituted flouranthene derivatives ; redox potentials ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ---The oxidation and reduction potentials of a series of related even non-alternant derivatives of 7,14-disubstituted acenaphth[1,2-k] fluoranthenes, and also fluoranthene, 7,10-diphenylfluoranthene and 8,9-dihydrodiindeno[1,2-j;2′,1′-] fluoranthene, were determined in organic solvents by cyclic voltammetry. The effects of steric hindrance on conjugation of the substituents with the central polycyclic aromatic hydrocarbon nucleus were evaluated. The semi-empirical molecular orbital calculation programs OMEGAMO, Extended Hückel, AM1 and PM3 were used to obtain optimal geometries and calculated HOMO and LUMO energies. As a further refinement, COSMO solvation was included in the AM1 calculations. The redox properties were correlated with data derived from the various semi-empirical calculations and the quality of these correlations is discussed. Inclusion of solvation energies in the computed molecular orbital energies results in a significant improvement in the correlation between observed and calculated oxidation potentials. © 1997 John Wiley & Sons, Ltd.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 17 (1996), S. 289-297 
    ISSN: 0192-8651
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: In spite of much work on path-following methods, a solid mathematical foundation (especially convergence conditions and their practical measures) are lacking in most cases. In our previous articles the general theory of a new global searching procedure, the dynamically defined reaction path (DDRP) method, its rigorous mathematical formulation, the algorithm, a practical computation program, and some applications to abstract mathematical functions and simple chemical examples have been presented. In this article we give a short theoretical description and some practical criteria and measures for the convergence of the method and illustrate the principles and uses by numerical mathematical and chemical examples. © 1996 by John Wiley & Sons, Inc.
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