Force field comparison and thermodynamic property calculation of supercritical CO2 and CH4 using molecular dynamics simulations

Thermodynamic properties of carbon dioxide and methane have been calculated under supercritical conditions up to 900 K and 100 MPa using isothermal-isobaric molecular dynamics simulations and the multistate Bennett acceptance ratio (MBAR) technique. Seven different carbon dioxide force fields (two single-site models, three rigid three-site models, and two fully flexible three-site models) were considered for preliminary density calculation. Those showing better accuracy when compared to experimental results were used to calculate the volume expansivity, isothermal compressibility, isobaric and isochoric heat capacities, Joule-Thomson coefficient, and speed of sound. The same properties were also calculated for methane using two different single-site models. The results show that force fields originally parameterized and optimized to reproduce vapor-liquid coexistence curves may be able to give accurate predictions of other thermodynamic properties in an extended temperature and pressure range. The results obtained with molecular simulations are generally more accurate than predictions with the Peng-Robinson equation of state, especially near critical conditions and at high pressures. Furthermore, the MBAR technique is successfully applied to improve the accuracy of results, decrease calculation uncertainties and reduce the number of simulations required to provide reliable property predictions over a range of temperatures and pressures. Recommendations are made as to which force fields are most accurate for the set of properties computed here. (C) 2014 Elsevier B.V. All rights reserved.