The Phase Diagrams of Simple Fluids: Ar, Kr, and CH4 Using Temperature Dependent Interaction Parameters

Based on our past results, it is hypothesized that the use of temperature dependent interaction parameters (TDIP), instead of using temperature independent interaction parameters (TIIP), may lead to improvement in the prediction of the vapor-liquid coexistence curve. Published compilations of the second virial coefficient were used to fit a simple two parameter temperature dependent model for the collision diameter and well depth parameters of the Lennard-Jones potential energy model. Subsequently, vapor-liquid coexistence curves for argon, krypton, and methane were simulated in the NVT Gibbs ensemble Monte Carlo (GEMC). The simulations were carried out using both TDIP and TIIP. The critical temperatures and densities were determined using the Ising-scaling model. The results using TDIP produce, in general, more accurate phase diagrams compared to the diagrams generated using TIIP. The root mean square deviation (RMSD) is reduced by 42.1% using TDIP for argon. Similar reductions in RMSD were observed for Kr and methane. Also, there was no significant difference between the results obtained using TDIP and the highly accurate and computationally demanding phase diagrams based on three body contributions for argon. The phase diagram in reduced units for temperature dependent interaction parameters is a family of curves rather than a universal curve such as the case with TIIP.