Vapor-liquid equilibria of square-well spheres

Results of molecular dynamics (MD) simulations on square-well fluids with λ=1.25, 1.375, 1.5, 1.75, and 2.0 are presented. The calculation of vapor-liquid equilibrium was performed by isochoric integration of the liquid NVE data to obtain the free energy of the liquid and equating this to the vapor free energy from a modified virial equation. The saturation pressure was investigated and compared with that from Monte Carlo simulation and second-order analytical perturbation theory. The vapor pressures from the isochoric integration technique are shown to be smoother than previous results, permitting accurate estimation of the effect of the square-well width on acentric factor. With the saturated properties from molecular dynamics, the f value used in Kofke’s Gibbs-Duhem integration was calculated and was found to be nearly constant. The related integration of the Clapeyron equation was implemented as a check on thermodynamic consistency. Vapor pressures presented here are consistent to within 2%.