A computational study of metastability in vapor-liquid equilibrium

Computer simulations are ideally suited to study systems under arbitrary constraints; hence they are useful for the investigation of metastability. Different types of constraints were applied to the three-dimensional Lennard-Jones fluid in the vapor-liquid coexistence region. Constraining the magnitude of allowed density fluctuations (restricted ensemble) has little effect on the equation of state and on phase equilibrium predictions for reduced temperatures lower than 0.95. Thermodynamic integrations along constrained and unstable paths are in good agreement with chemical potential calculations, indicating that imposing the density constraint does not violate microscopic reversibility. Restricted ensemble calculations were also used to calculate the width of the transition region where the mechanism of phase separation in the superheated liquid changes from nucleation to spinodal decomposition. The width of this region decreases as the temperature is reduced away from criticality. Free energy barriers to isotropic compression were used to determine the width of the transition region from nucleation to spinodal decomposition in the supercooled vapor. This transition region also becomes narrower as the distance from the critical point increases. The pressure of the deeply superheated liquid was found to be sensitive to the maximum size of voids that are allowed to form.