AB-INITIO METHANE DIMER INTERMOLECULAR POTENTIALS

Two purely ab initio non-bonded potential functions for methane dimer interactions in the MM3 framework have been established. The parameter fitting was performed to 130 carefully selected configurations. Intermolecular energies and forces have been evaluated at the second-order Moller-Plesset perturbation theory level (MP2) with a medium size basis set. Since the resultant energies were found to be too shallow, an empirical correction procedure to the existing MP2 potential energy surface (PES) was carried out to obtain MP2 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 PES. Both resultant potentials predicted reasonable agreement with experimental properties, which included methane and butane virial coefficients, liquid methane and butane densities and heats of vaporization, liquid methane C-C radial distribution functions, crystal unit cell volumes, and heat of sublimation data. The role of more sophisticated potential functions has also been examined.