Ab initio pair potentials and phase equilibria predictions of halogenated compounds

Abstract

Ab initio pair potentials are developed for two fluorinated compounds, hydrogen fluoride and methyl fluoride, using symmetry-adapted perturbation theory (SAPT) by considering several hundreds of configurations between a pair of molecules. The interaction energies were fitted to a site–site pair potential model. Also, a pairwise interaction model for a chlorinated compound, hydrogen chloride, was taken from the literature. These pair potentials were then used to predict the second virial coefficients and, using Gibbs ensemble Monte Carlo (GEMC) simulations, the vapor–liquid equilibria of each of the compounds. The properties of methyl fluoride and hydrogen chloride were found to be very well described with the ab initio pair potentials, resulting in predicted second virial coefficients, liquid densities and vapor pressures in good agreement with experimental data. Estimated critical temperatures and densities deviate by only few percent from reported values. However, for hydrogen fluoride, the ab initio pair potential results in a poor description of its phase behavior, but this is not unexpected since hydrogen fluoride is a strong hydrogen-bonding fluid and multi-body effects play a large role in the molecular interactions.