Molecular mechanics parameters for the homologous series of fluorine-substituted methanes, CHxF4-x, are derived from ab initio Hartree-Fock surfaces. Ab initio intermolecular potential energy surfaces are calculated using a 6-31+G* basis set and include correlation using second-order Moller-Plesset perturbation theory. A least squares fit of the ab initio surface to a standard molecular mechanics potential function, including Lennard-Jones interactions plus partial charges, is then performed. The thermodynamic properties of the resulting molecular mechanics potential are calculated using conventional molecular dynamics simulations and compared to experimental results. Additional fine-tuning of the molecular mechanics potential is then performed to optimize the agreement with experimental results. The effect of including high-energy configurations in the fit is systematically investigated.