MOLECULAR MECHANICS PARAMETERS FOR FLUORINE-SUBSTITUTED METHANES FROMAB-INITIO QUANTUM CALCULATIONS

Molecular mechanics parameters for the homologous series of fluorine-s ubstituted methanes, CHxF4-x, are derived from ab initio Hartree-Fock surfaces. Ab initio intermolecular potential energy surfaces are calcu lated using a 6-31+G basis set and include correlation using second-o rder 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 per formed. The thermodynamic properties of the resulting molecular mechan ics potential are calculated using conventional molecular dynamics sim ulations 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-ene rgy configurations in the fit is systematically investigated.