Tracing coexistence lines in multicomponent fluid mixtures by molecular simulation

Novel formulations of Gibbs-Duhem integrations are proposed to trace phase diagrams for multicomponent fluid mixtures, including generalized bubble-de w lines, whereby the composition of one of the phases is specified, but may not be constant along the integration path. The algorithms presented in th is work constitute an extension of a recently proposed pseudo-ensemble fram ework. Different types of phase equilibrium calculations with open ensemble s are cast into a unique formulation, which leads to a simple and flexible integration scheme for stepping along a coexistence line. This approach is applied to trace phase diagrams for systems described by realistic force fi elds, including hydrocarbon mixtures. A variant of a Gibbs ensemble that pe rmits simulation of bubble-dew point pressures is also implemented. (C) 199 9 American Institute of Physics.