Computing phase equilibria by parallel excluded volume tempering

We present a Monte Carlo scheme for the computation of phase equilibria at high densities. At these high densities, all conventional simulation techni ques that rely on insertions and deletions of particles, e.g., the Gibbs en semble technique, will have problems because the acceptance probability for these moves is very low. Furthermore, the efficiency of these methods stro ngly depends on the complexity of the system, e.g., degree of polymerizatio n and branching of the components. Our new method is based upon simulating a path of independent systems in the grand-canonical ensemble. Each system has a slightly different interaction potential, ranging from a full exclude d volume potential to an ideal gas, as well as different imposed chemical p otentials of each component. This path is constructed in such a way that th e average number of molecules of a specific component per system is constan t along the path. To sample all systems of the path efficiently, we apply a parallel tempering procedure to exchange configurations of two adjacent sy stems. The advantage of these exchanges is that, for the full excluded volu me system, one does not have to rely on particle insertions and deletions i n this system to sample the full phase space, but rather on particle insert ions and deletions in systems with soft interactions. Without excluded volu me interactions, the acceptance of insertions is independent of molecular s ize and shape; hence our method does not suffer from the problems of the co nventional methods. We have tested our method for very simple systems (Lenn ard-Jones particles) and found exact agreement with Gibbs ensemble simulati ons. For these simple systems the conventional techniques to compute phase equilibria are much more efficient. However, we expect that for long chain molecules this situation will be reversed. (C) 2001 American Institute of P hysics.