Ab initio pair potential and phase equilibria predictions for the refrigerant methyl fluoride

Ab initio pair interaction energies for methyl fluoride (CH3F) were calculated using symmetry adapted perturbation theory with the aug-cc-pVDZ basis set plus bond functions for a large number of configurations of a pair of methyl fluoride molecules in order to obtain a good representation of the two-body potential energy surface. These interaction energies were used to develop a site-site intermolecular potential that accurately reproduced the calculated energies and had the correct asymptotic behaviour at long range. This pair potential was used in Gibbs ensemble Monte Carlo simulations to predict the phase behaviour of methyl fluoride. The predicted equilibrium properties with the ab initio pair potential are in good agreement with the experimentally measured phase boundary, and give an estimated critical point in excellent agreement with measured values. Multi-body effects were then added to the pair potential using a polarizable model. The small changes to the phase behaviour that resulted established that pairwise interactions account for most of the molecular interactions in CH3F.