THE CONDITION OF MICROSCOPIC REVERSIBILITY IN GIBBS ENSEMBLE MONTE-CARLO SIMULATIONS OF PHASE-EQUILIBRIA

The condition of microscopic reversibility, also referred to as detail ed balance, is examined in the context of Monte Carlo simulations in t he Gibbs ensemble. The technique is used widely in the simulation of p hase equilibria for liquids and their mixtures, and represents an inva luable tool in the area. The two coexisting phases are simulated as se parate subsystems by performing three distinct Monte Carlo moves which include random displacements of particles in each subsystem, random c hanges in volume, and random transfers of particles between the two su bsystems. Here, the particle transfer step of the Gibbs ensemble techn ique, as commonly implemented, is shown to be reversible. Other valid reversible criteria are presented for pure fluids and mixtures. The va pour-liquid equilibria of the pure square-well fluid with a range of l ambda = 1.5 are examined with the various criteria. As expected, the c hoice of criteria makes little difference for pure fluids. The results are also presented of liquid-liquid immiscibility for a symmetrical s quare-well mixture with range lambda = 1.5 in which the unlike interac tions are purely repulsive. For this mixture the various reversible al gorithms for particle transfers give essentially equivalent results, a lthough the efficiency in sampling phase space is sometimes quite diff erent.