Analysis and Improvement of Intermolecular Potentials for Hydrocarbons in Molecular Mechanics Force Fields.
To improve upon non-bonded force fields for hydrocarbons, a "toolbox" providing the link between the potentials and experimentally or computationally available observables has been developed. The relevant properties obtained by the programs in the toolbox are as follows: (i) ab initio potential energies of hydrocarbon dimers using Moller-Piesset perturbation theory through second order (MP2); (ii) second virial coefficients of both fixed and rotationally flexible molecules in a wide temperature range; (iii) enthalpies of vapourization and densities as well as structural properties of liquids by means of the Monte Carlo method in the isothermal -isobaric ensemble; (iv) enthalpies of sublimation, lattice dimensions, and unit cell volumes of crystals. Novel parametrization procedures have been devised to obtain close agreement between calculated and experimental properties. Two MP2 non-bonded potential functions for methane dimer interactions in the MM3 framework have been established. Since the resultant MP2 energies were found to be too shallow, an empirical correction procedure was carried out to obtain energies in a very large basis set. The first of the two emerging parameter sets was obtained by fitting to MP2 forces, while the second is based on the corrected MP2 potential energy surface. Both resultant potentials predicted reasonable agreement with experimental properties. A critical examination of ten existing alkane non-bonded empirical potential functions covering the widest range of molecular mechanics potential functions has been performed by computing the aforementioned properties. Ten new parameter sets have also been obtained based on various criteria to fit to ab initio and empirical properties. Fitzwater's and Bartell's modified Urey-Bradley potential, and a reparameterized version of Allinger's MM3 function, termed MM3mc, have emerged as the two most reliable potentials. The relationships between properties in different phases have been explored. An extensive characterization of six existing non-bonded analytical 12-site potential functions for benzene has been performed. The 12-6 potential of Jorgensen and Severance emerged as the best effective two-body potential in condensed phases, while the exp-6 potentials of Williams and Starr proved to be the most consistent in all three phases. Two new parameter sets have been obtained in the framework of the MM3 potential, and one of them represents significant improvement over the original MM3 parameters. An analysis of the 12-6 and exp-6 potential forms is given. To validate the most promising parameter sets from the alkane and aromatic studies, respectively, they have been combined and utilized to obtain crystal properties for two fullerenes, and adsorbtion energies between hydrocarbon molecules and graphite.
- Pub Date:
- January 1995
- Chemistry: Physical; Physics: Molecular