摘要:

AbstractThe geometry of the ground states of the isomers and transition state for the systems HCN ⇌ HNC and BCN ⇌ BNC have been investigated using a wide variety of basis sets, both at the self‐consistent‐field (SCF) level and including correlation at the second‐, and third‐order Møller–Plesset (MP2 andMP3) levels. The barrier to isomerization and the isomerization energy were shown to be strongly dependent on the basis set and method, particularly for

ISSN:0192-8651    

DOI:10.1002/jcc.540050102

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractAb initiocrystal orbital calculations have been performed on regular polyethylene chains applying basis sets of minimal and double‐zeta quality. Relative stabilities of periodic all‐trans, all‐gauche, and alternating trans–gauche conformers have been evaluated, including extensive geometry optimization. Potential curves for a simultaneous rotation around CC single bonds from the all‐trans to the all‐gauche conformation have been computed applying the rigid‐rotor approximation, the flexible‐rotor approximation, and an additional reoptimization of CC distances. A rigid‐rotor potential curve from the all‐trans to the alternating trans‐gauche conformation has been computed as well. Results obtained are compared withab initiocalculations on butane and pentane and with semiempirical and empirical force‐fi

ISSN:0192-8651    

DOI:10.1002/jcc.540050103

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe structure of an isolated, infinite polyoxymethylene chain has been investigated with the aid of theab initiocrystal orbital method applying a basis set of double‐zeta quality. Restricting the primitive unit cell to a single CH2O group, conformational potential curves as a function of the torsional angle have been evaluated. Only a single minimum closely corresponding to an all‐gauche structure was detected. The all‐trans conformation is a maximum on the energy curve for simultaneous rotation around CO single bonds. Detailed geometry optimization in the vicinity of the all‐gauche conformation led to the following structure:rCO=rOC= 1.425 Å,rCH= 1.072 Å, ∠HCH = 111.7°, ∠OCO = 110.9°, ∠COC = 115.1°, and τOCOC= 70.75°. The computed torsional angle τOCOClies midway between the hexagonal (78.2°) and the orthorhombic (63.5°) modification

ISSN:0192-8651    

DOI:10.1002/jcc.540050104

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractWe consider the problem of predicting the mode of binding of a small molecule to a receptor site on a protein. One plausible approach, given a rigid molecule and its geometry, is to search directly for the orientation in space that maximizes the degree of contact. The computation time required for such a naive procedure is proportional ton3m3, wherenis the number of points in the site where binding can occur, andmis the number of atoms in the ligand. We give an alternative, combinatorial approach, in which only “contact–no‐contact” criteria are considered. We relate this problem to the well‐known combinatorial problem of finding cliques in a graph and show that we can use a solution to the clique problem not only to solve our original problem, but also the problem of avoiding energetically unfavorable matches. Our experience with this method indicates that the computation time required is proportional tonm2.8, with a lower constant of proportionality than that of the naive

ISSN:0192-8651    

DOI:10.1002/jcc.540050105

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractSemiempirical molecular orbital calculations by theMNDOmethod are reported for sulfur rings and chains S3through S8, for the corresponding dications, S 22+through S 82+, and for S 4+. TheMNDOmethod seems quite successful in predicting the geometries of neutral catenated sulfur molecules, even the unusual bond‐length alternation and extent of coplanarity incyclo‐S7. In contrast to hydrocarbon rings, for which its prediction of strain is erratic,MNDOis consistent in its calculated strain energies in small cyclosulfur rings; unfortunately all the strain energies are overestimated by 70%. As a consequence of this error, the method must be considered unreliable in its predictions of structures for the dications S n2+, since many of these ions could potentially exist as strained bicyclic systems. In addition,MNDOappears to have difficulty handling long, partial SS σ bonds, as are found to occur in S 82+. It may be for this reason thatMNDOpredicts, apparently incorrectly, that the open‐chain isomers of S n2+are more stable than are any of the cyclic forms, at least for S 52+to S 82+. With respect to neutral Snmolecules, however, theMNDOpredictions appear more reliable thanab initiomolecular orbital (MO) calculations using small basis sets without polarization functions and without configuration interaction (CI). However,MNDOapparently underestimates by about a factor of two the strength of the three‐electron π bonds present in the terminal links o

ISSN:0192-8651    

DOI:10.1002/jcc.540050106

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe Floating Point Systems, Inc. Model 164 Attached Processor (FPS‐164) is a high‐speed, pipelined, parallel processor designed for large‐scale scientific computation. Benchmark studies of operations common in quantum chemistry codes are discussed and the performance of the FPS‐164 is compared with other commonly available computers. A complete system of electronic structure codes has been implemented on the FPS‐164 using the Fortran‐77 cross‐compiler and calls to optimized vector and matrix routines. The conversion of a generalized valence bond (GVB) code illustrates the strategy adopted to adapt Fortran codes to the FPS‐164. A typical production example, a large scale (GVB) and configuration interaction calculation on the vinyl radical, shows a net throughput equivalent to nearly nine VAX 1

ISSN:0192-8651    

DOI:10.1002/jcc.540050107

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractExtension of the inhomogeneous continuum solvent model to prolate spheroidal cavity systems in the context of Kirkwood–Westheimer substituent‐reactivity theory is described. Reasonable effects attributable to electrical saturation and electrostriction, which are modeled by relatively simple spatial dielectric functions outside the solute–solvent boundary, may be demonstrated. It is also shown that choices of proper (i.e., nonaveraged) location of the interacting sites and magnitude of substituent dipole moments are comparably important to the quality of theoretical predi

ISSN:0192-8651    

DOI:10.1002/jcc.540050108

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe degrees of conformational freedom of polyL–Dβ‐helical chain are analyzed consistent with the helical parameters of gramicidin A structure. From conformational energy calculations, “helical librations” that can be sustained by this structure are described and the energy of libration as a function of the cavity size is presented. Two different modes of conformational change are identified corresponding to librations of allL–D‐peptide units or allD–L‐peptide units while retaining the helical parameters. Such helical librations are considered relative to conformational perturbations due to the presence of an io

ISSN:0192-8651    

DOI:10.1002/jcc.540050109

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractA simple computational scheme for estimating the electrostatic potential about molecular models of moderate size is given. The large amount of calculations required for the evaluation of the hypersurface lends itself to treatment by high speed, unconventional computing machines. The essence of these calculations lies in Coulombic interactions that are computed between hypothetical proton test probes positioned in a gridded region surrounding the model and the partial electrostatic charges (CNDO/2) of each atom in the model. A specific scientific application is discussed which involves the recognition of amino acids and nucleotide bases. Three different evaluations of the potential hypersurface within the context of this approach were made. The first was performed on a VAX 11/780 which is a general purpose machine widely used in the scientific community; the second was performed using a pipelined Vector Processor, the FPS AP‐120B; and the third by a processor array, the ILLIAC‐IV. A comparison of the architectures and processing speeds of each class of machines is made. The computing power observed is consistent with the design and purpose of each machine. Also discussed are methods for displaying the vast amount of data that result from such calculations. It is determined that computer graphics offers an effective means for extracting information from large amounts of data. Finally, the scientific value of the calculations are briefly discussed. If caution is applied to interpreting the results, then the electrostatic potential (EP) mappings can be useful in identifying sites of potential chemical interacti

ISSN:0192-8651    

DOI:10.1002/jcc.540050110

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe use of symmetry coordinates (SC) in geometry optimization is discussed. A computer program incorporating the use of sc, together with analytical calculation of the gradient and quadratic acceleration, is described. Also reported are careful test results on a series of small molecules and typical results with a long series of molecules up to quite large size (40–60 atoms

ISSN:0192-8651    

DOI:10.1002/jcc.540050111

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY