Method for liquid–vapor coexistence curves by test-particle insertions in the canonical ensemble

Abstract

We propose a new method to obtain liquid–vapor coexistence curves from microscopic information of interatomic potentials. The basic idea underlying our method is to calculate chemical potentials by test-particle insertions during canonical simulations and to determine the equilibrium vapor pressure. The advantage of our method is that we perform simulations at a fixed volume so that the undesirable volume fluctuations do not occur even near the critical point between the liquid and the vapor phase and consequently we make the simulations stable. This is an outstanding advantage when compared to the NpT plus test-particle method in which the simulations are difficult near the critical point because of large volume fluctuations and frequent phase transitions. We apply our method to the Lennard-Jones system. We determine the critical point more accurately and the coexistence curve closer to the critical point than the result of the NpT plus test-particle method. We also show that the law of rectilinear diameter is fulfilled even at temperatures very close to the critical point.