Methane adsorbed on graphite. I. Intermolecular potentials and lattice sums

The potential-energy surface for methane adsorbed on the basal plane of graphite is determined from two different atom-atom intermolecular interaction models. Static lattice sums are calculated for solid adlayer films in which the structure is varied in a systematic search for potential-energy minima. The potential-energy barriers to desorption, translations, and rotations are calculated, and a structure with two molecules per unit cell is investigated. Estimates of the translational, librational, and vibrational zero-point energies are given. Comparisons to experiments are discussed.