Hydrogen fluoride phase behavior and molecular structure: Ab initio derived potential models

Abstract

Several variations of ab initio based molecular models for hydrogen fluoride (HF) are examined by Monte Carlo molecular simulation to determine their bulk-phase properties. The models are taken from the literature, and represent fits of functional forms to the potential energy surface of the HF dimer as given by ab initio computational chemistry calculations. For one of these models, we examine three variations for bulk-phase modeling. In particular, we consider first the effect of including versus neglecting an Ewald sum for the long-range dipole–dipole interactions; second, we examine a modification of the form for the short range repulsive region of the potential; and third, we add three-body contributions to the energy via an available 12-dimensional potential for the trimer, again representing a fit to ab initio energy calculations. The simulations examine the density (via isothermal–isobaric simulation) and radial distribution function (via canonical–ensemble simulations) each at two state points where corresponding experimental data are available. We also examine vapor–liquid coexistence properties, considering the saturation densities, heat of vaporization, and vapor pressure from 225 K to states approaching (but not closely) each model’s critical point. Inclusion of the three-body energy is the only variation that has any beneficial effect on the radial distribution function as compared to experiment, and this variation also gives good results for the vapor pressure, and significantly raises the critical point toward the experimental value. However this model also grossly overestimates the liquid-phase coexistence density. In almost all regards none of the models or variations can be considered to give a satisfactory representation of the bulk-phase behavior. Improvements to the models require more careful attention to the balance between repulsive and attractive pair interactions at short range.