Communication: Tracing phase boundaries via molecular simulation: an alternative to the Gibbs-Duhem integration...

Precise simulation of phase transitions is crucial for colloid/protein crystallization for which fluid-fluid demixing may be metastable against solidification. In the Gibbs-Duhem integration method, the two coexisting phases are simulated separately, usually at constant-pressure, and the phase boundary is established iteratively via numerical integration of the Clapeyron equation. In this work, it is shown that the phase boundary can also be reproduced in a way that avoids integration of Clapeyron equations. The two phases are simulated independently via tempering techniques and the simulation data are analyzed according to histogram reweighting. The main output of this analysis is the density of states which is used to calculate the free energies of both phases and to determine phase coexistence. This procedure is used to obtain the phase diagram of a square-well model with interaction range 1.15σ, where σ is the particle diameter. The phase boundaries can be estimated with the minimum number of simulations. In particular, very few simulations are required for the solid phase since its properties vary little with temperature.