Journal of Computational Chemistry

Mercau, N.; Aroca, R.; Robinson, E. A.; Aron, J.; Bunnell, J.; Ford, T. A.

doi: 10.1002/jcc.540050503pmid: N/A

The force constants and compliance constants of methyl, silyl, and germyl fluoride, chloride, bromide, and iodide have been calculated by the iterative consistency method. Using the force fields so obtained, centrifugal distortion constants, Coriolis coupling constants, and mean amplitudes of vibration have been computed and compared with experimental data, where available.

Hall, David; Pavitt, Nicola

doi: 10.1002/jcc.540050504pmid: N/A

A number of force fields of the molecular mechanics type have been tested for their ability to represent as an energy minimum, the observed crystal structure for three cyclic hexapeptides, cyclo‐(‐Ala‐Ala‐Gly‐Gly‐Ala‐Gly‐), cyclo‐(‐Ala‐Ala‐Gly‐Gly‐Ala‐Gly‐), and cyclo‐(‐D‐Ala‐D‐Ala‐Gly‐Gly‐Gly‐Gly‐). The most effective force field tested was that recently proposed by Kollman and co‐workers, notwithstanding its use of “united” atoms for CH, CH2, and CH3 groups. Fields proposed by Levitt, and adaptations of that of Scheraga and co‐workers, were also effective. Force fields in which hydrogens bonded to electronegative atoms were not specified explicitly were less accurate in representation.

Siam, K.; Klimkowski, V. J.; Ewbank, J. D.; Schäfer, Lothar; Van Alsenoy, C.

doi: 10.1002/jcc.540050505pmid: N/A

The structures and conformational energies of several conformations of propanoic acid, 2‐methylpropanoic acid, and butanoic acid were determined by geometrically unconstrained ab initio gradient geometry refinement on the 4‐21G level. The OCCC torsional potentials of propanoic acid and butanoic acid are found to be practically identical. There are energy minima at 0° and 120°, and maxima in the 60° region and at 180°. In 2‐methylpropanoic acid there are energy minima at HCCO dihedral angles of 0° and 120°, and maxima at 60° and 180°. The exact positions of the maxima and minima of the HCCO torsional potential of 2‐methylpropanoic acid are found to be predictable from propanoic acid rotational‐potential parameters. Some conformationally dependent, local geometry trends are discussed.

Leugers, M. A.; Seliskar, C. J.

doi: 10.1002/jcc.540050506pmid: N/A

A Fortran‐77 computer program has been written which calculates the rotation–torsion profile of a vibronic transition using an asymmetric top/free internal rotor Hamiltonian. The program is applicable to any molecule that is composed of a frame portion of C2v symmetry a free rotor portion of C3v symmetry. Rotation–torsion bands of A‐, B‐, and C‐type contours may be calculated within an assumed Boltzmann distribution of rotation–torsion level populations or within a specified non‐Boltzmann distribution of torsional level populations.

Zielinski, Theresa Julia; Poirier, Raymond Alcide

doi: 10.1002/jcc.540050507pmid: N/A

Structures and relative energies were obtained for the hydrogen bonded dimers of formamide and formamidic acid using the 3‐21G basis set. A double proton transfer transition state is claimed to link these two dimers. While the structure of the transition state was intermediate between those of the two dimers, the energy was only 7.6 kJ/mol greater than the less stable formamidic acid dimer. The activation energy from the formamide dimer side of the reaction was found to be 125 kJ/mol of dimer. A similar transition state was found for the amidine dimer system. The activation energy for this model reaction was found to be 66.9 kJ/mol of dimer.

Rappé, A. K.

doi: 10.1002/jcc.540050508pmid: N/A

Examples of the types of algorithmic modification needed to achieve substantial enhancement for quantum chemical codes on vector processors such as the Cyber 205 are presented. Specific examples include matrix transformations, 4‐index integral transformations, and general multiconfiguration–self‐consistent‐field (MC‐SCF) codes.

Dewar, Michael J. S.; Fox, Marye Anne; Campbell, Kay A.; Chen, Chia‐Chung; Friedheim, James E.; Holloway, Mary K.; Kim, Shi Choon; Liescheski, Phillip B.; Pakiari, Ali M.; Tien, Tze‐Pei; Zoebisch, Eve G.

doi: 10.1002/jcc.540050509pmid: N/A

The energies of the lowest singlet (S1) and triplet (T1) states of 28 molecules have been calculated by the “half‐electron” (MNDO‐HE) and spin‐unrestricted (UMNDO) versions of MNDO. While most of the calculated values are too negative, because of overestimation of the correlation energy in MNDO‐HE and UMNDO, the errors are systematic and depend in an understandable way on the nature of the molecular orbitals (MOS) involved. When appropriate corrections are applied, the calculated energies agree with experiment almost as well as they do for ground states. This justifies the use of MNDO‐HE or UMNDO for studies of excited state processes.

Hopfinger, A. J.; Pearlstein, R. A.

doi: 10.1002/jcc.540050510pmid: N/A

A set of procedures and guidelines are presented for the estimation of bond length, bond angle, and torsional potential constants for molecular mechanics force fields. The force field constants are ultimately derived by “subtracting” nonbonded molecular mechanics energies from corresponding molecular orbital energies using a model compound containing the chemical structure to be parameterized. Case study examples of bond length, bond angle, and torsional rotation force field parameterizations are presented. A general discussion of molecular mechanics force field parameterization strategy is included for reference and completeness. Finally, a curve‐fitting program to generate force field parameters from raw data is given in Appendix I.