Free Energy Calculations

Abstract

The techniques that can be used to study phase diagrams numerically depend on the character of the phase transitions. In particular, there is quite a difference among the tools used to study first-order phase transitions and those applied to analyze critical phenomena near continuous phase transitions. The most straightforward way to study phase behavior by computer simulation is to change the temperature or pressure of a given model system and then to simply wait for a phase transformation to occur. There exist several schemes to study phase coexistence without creating an interface. In fluids, the best-known method is the Gibbs ensemble method. A direct method for studying phase coexistence that does not suffer from this drawback is the so-called Gibbs-Duhem integration method. This method is designed to trace out a two-phase coexistence curve, once one point on that curve is known. There exist several techniques for measuring the chemical potential of a given species in a single Monte Carlo (MC) or Molecular Dynamics (MD) simulation. A particularly simple and elegant method for measuring the chemical potential μ of a species in a pure fluid or in a mixture is the particle insertion method.