An Improved Force Field for the Prediction of the Vapor−Liquid Equilibria for Carboxylic Acids

An improved united-atom force field, based on a Lennard-Jones plus point charge functional form, for carboxylic acids is proposed. Point charges are determined from a Mulliken population analysis of ab initio calculations performed at the MP2 level of theory with the 6-31+g(d,p) basis set. The Lennard-Jones well depth and size parameters for the carboxyl carbon interaction site were determined by fitting to single-component vapor−liquid equilibrium data for acetic acid, while the remaining Lennard-Jones parameters were taken from the TraPPE-UA force field. Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble were used to determine the vapor−liquid coexistence curves, vapor pressures, and critical points predicted by the new force field. The new force field was found to describe accurately the phase equilibria of acetic acid, with mean unsigned errors in the saturated liquid density of less than 1.6%. The critical temperature and density predicted by the new force field were within 1% of experimental values. Additional calculations were performed in the grand canonical and Gibbs ensembles to determine the coexistence properties predicted by the optimized potentials for liquid simulations united-atom (OPLS-UA), all-atom (OPLS-AA), and CHARMM force fields. Simulations in the isobaric−isothermal ensemble were used to determine selected radial distribution functions predicted by the four force fields at 300 K and 1 bar.