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Grand canonical simulation of phase behaviour in highly size-asymmetrical binary fluids

We describe a Monte Carlo scheme for the grand canonical simulation study of fluid phase equilibria in highly size-asymmetrical binary mixtures. The method utilizes an expanded ensemble in which the insertion and deletion of large particles is accomplished {\em gradually} by traversing a series of states in which a large particle interacts only partially with the environment of small particles. Free energy barriers arising from interfacial coexistence states are surmounted with the aid of multicanonical preweighting, the associated weights being determined from the transition matrix. As an illustration, we present results for the liquid-vapour coexistence properties of a Lennard-Jones binary mixture having a 10:1 size ratio.