Efficient volume changes in constant-pressure Monte Carlo simulations

A new method for calculating the energy change resulting from an affine deformation of the system in constant-pressure Monte Carlo simulations is presented. The energy changes accompanying particle displacements are calculated exactly, but those accompanying volume changes are approximated. The latter energy is estimated from a Taylor expansion whose coefficients depend on the energetics of the system in its predeformed state. This' method does not require an assumption of conformal potentials and is as readily applied to molecular systems and mixtures as to simple monatomic fluids. The expansion employed is not self-truncating and two ansatzen for this purpose are considered. The results obtained using these methods are compared with those from constant-pressure simulations using exact, time-consuming summations of the pair interactions in the affinely deformed system. Because of the additional overhead required, the method is not especially efficient for ordinary NPT simulations. However, it is found to be an order of magnitude faster than conventional constant-pressure algorithms when frequent volume changes are required or when simulating phase transitions using the Gibbs-Duhem integration technique.