The Lennard-Jones 12-6 potential is widely used to model the dispersive interactions between atoms in molecular simulations. While generally successful in reproducing the thermophysical properties for a wide range of fluids, some deviations between experiment and simulation have been observed. For example, recent work in force field development has shown that typical united-atom force fields for alkanes, using the standard LJ 12-6 potential, can be parameterized to reproduce either the saturated liquid densities, or the vapor pressure to within 1% of experimental values, but not both at the same time [1, 2]. In this work, the effect of the repulsive exponent on the predicted saturated densities, vapor pressures and accentric factors is determined for LJ n-6 spheres and diatomics using histogram-reweighting Monte Carlo simulations in the grand canonical ensemble. The results show that the acentric factor is a strong function of the repulsive exponent. New n-6 potential parameters are optimized for n-alkanes, perfluorocarbons and alcohols that show an improved reproduction of the vapor pressure over existing united-atom force fields, with no loss in accuracy in the prediction of saturated liquid densities.

References: [1] M. G. Martin, J. I. Siepmann; J. Phys. Chem. B 102 (1998) 2569-2577 [2] S. K. Nath, F. A. Escobedo, J. J. de Pablo; J. Chem. Phys. 108 (1998) 9905-9911