Intermolecular pair and trimer potentials for methyl fluoride

Ab initio calculations of the pair and trimer interaction energy of methyl fluoride are reported in various approach configurations. These are based on the 6 - 31G and 4 - 31G basis sets of the Gaussian 76 program of Pople and co-workers. The potential energy surface of pair interaction is compared with that of Copeland and Cole, based on gas viscosity, second pressure virial and second dielectric virial data, and with an unoptimised 5 × 5 atom-atom potential. The “experimental” Copeland/Cole potential and the ab initio calculations are more often in better agreement for different dimer configurations than with the atom-atom potential. Non-pair-additivity energy in the minimised cyclic trimer of CH3F is of the order of 10% of the total trimer interaction energy and is expected to become more important in bigger (multimer) clusters.The results indicate that a parameterisation of the ab initio results or alternatively a straightforward use of the Copeland/Cole form is more likely to be realistic in a molecular dynamics simulation of CH3F when using the assumption of pair interaction. However it is best to avoid this assumption in the manner of Finney et al.