Monte Carlo simulation of free energy for the solid-liquid equilibrium of methane

Abstract

The thermodynamic properties of methane, particularly for solid-liquid equilibrium, are calculated by Monte Carlo simulation. For various potential models of methane, we explicitly calculated free energies and chemical potentials of the solid and liquid phases of methane by using the expanded ensemble method and the thermodynamic integration method. The Einstein-molecule method combined with the expanded ensemble method is used for the solid phase, and thermodynamic integration for the liquid phase. Coexistence properties such as melting temperature, entropy change and enthalpy change of melting are predicted and compared with experiment. Among the potential models studied, the OPLS-AA model shows the best performance in predicting the solid-liquid coexistence properties of methane. The melting temperature at zero pressure is predicted to be 92.6 K, in good agreement with the experimental value of 90.6 K. While other all-atom potential models reasonably predict the density of solid methane within an error of 5%, they tend to underestimate the melting temperature. The OPLS-AA potential model yields the most accurate value for the entropy change of melting, predicted to be 8.71 J/mol·K. This is within an error of 16%, compared to the experimental value of 10.4 J/mol·K. Also, the enthalpy change of melting is predicted to be 0.81 kJ/mol with an error of 14%, compared to the experimental value of 0.94 kJ/mol.