Coexistence calculation using the isothermal-isochoric integration method

Abstract

In this work, the isothermal-isochoric integration (ITIC) method is demonstrated as a viable method for vapor-liquid coexistence calculation by molecular simulation. Several tests are carried out to validate the method. The first group of tests utilizes self-consistent NIST REFPROP values to demonstrate that, in the absence of simulation uncertainties, the ITIC method yields coexistence values with less than 1% deviation for reduced temperatures of less than 0.85. The impact of various simulation specifications are then compared. Following our recommended simulation methodology, consistent results are achieved between the ITIC method, Gibbs Ensemble Monte Carlo (GEMC) method, and Grand Canonical Monte Carlo (GCMC) method for reduced temperatures of 0.6–0.85. The ITIC method proves to be much more effective compared to GEMC and GCMC methods for vapor-liquid coexistence calculations at reduced temperatures of 0.45–0.6, which are important for practical applications. It is shown that computational efficiency is often served best by applying the ITIC method for the entire temperature range rather than applying Monte Carlo (MC) methods for part of the range. Furthermore, the ITIC method lends itself to application with molecular dynamics (MD) as well as MC, advancing the prospect of simulation results that are quantitatively consistent across software platforms.