Direct determination of phase coexistence properties of fluids by Monte Carlo simulation in a new ensemble

TLDR

The study introduces a Monte Carlo method in a new ensemble to determine phase‑coexistence properties of multicomponent fluids from a single simulation. The approach runs simultaneous simulations in two regions, using random displacements, volume adjustments, and particle transfers to enforce pressure and chemical‑potential equality, and is tested on Lennard‑Jones fluids from near the triple point to near the critical point. Results agree well with literature across most temperatures, with minor discrepancies near the limits of the studied range.

Abstract

Abstract A methodology is presented for Monte Carlo simulation of fluids in a new ensemble that can be used to obtain phase coexistence properties of multicomponent systems from a single computer experiment. The method is based on performing a simulation simultaneously in two distinct physical regions of generally different densities and compositions. Three types of perturbations are performed, a random displacement of molecules that ensures equilibrium within each region, an equal and opposite change in the volume of the two regions that results in equality of pressures, and random transfers of molecules that equalize the chemical potentials of each component in the two regions. The method is applied to the calculation of the liquid-gas coexistence envelope for the pure Lennard-Jones (6, 12) fluid for several reduced temperatures from the vicinity of the triple point to close to the critical point (T∗ = 0.75 to T∗ = 1.30). Good overall agreement with previously available literature results is obtained, with some deviations at the extremes of this temperature range.