A direct method for determination of chemical potential with molecular dynamics simulations

Abstract

A new direct method for the determination of chemical potentials from molecular dynamics simulations is proposed. This paper focuses on development and testing of the method for pure, Lennard-Jones fluids. The proposed method uses a semipermeable membrane to mimic an osmotic experiment. With only a few particles designated as those to which the membrane is permeable, the chemical potential relative to that of an ideal gas can be determined for a fluid at virtually any realistic temperature and density, and only the equilibrium mole fraction of the permeable particles in the ‘mixture’ side of the cell is required. The chemical potentials obtained using the new osmotic method agree within the precision of the calculations with those obtained from the Lennard-Jones equation of state and values obtained from Widom's method. The new method allows determination of chemical potential at virtually any state point from a single simulation and should be readily extendable to structured molecules. A unique feature of the osmotic method is that phase separation occurs in the two-phase region, unlike ordinary molecular dynamics simulations.