Ab initio potential energy surface for methane and carbon dioxide and application to vapor-liquid coexistence

A six-dimensional intermolecular potential energy surface for a rigid methane (CH4) and carbon dioxide (CO2) dimer was developed from the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory. A total of 466 grid points distributed to 46 orientations were calculated from the complete basis set limit extrapolation based on up to aug-cc-pVQZ basis set. A modified site-site pair potential function was proposed for rapid representation of the high level ab initio calculations. A nonadditive three-body interaction was represented by the Axilrod-Teller-Muto expression for mixtures with the polarizability and the London dispersion constant of each molecule. Second to fourth virial coefficients of CH4 and CO2 mixtures were calculated using both the Mayer sampling Monte Carlo method and the present potential functions. The virial equation of state derived from these coefficients was used to predict the pVT values and showed good agreement with experimental data below 200 bar at 300 K. The vapor-liquid coexistence curves of pure CH4, CO2 and their mixtures were presented with the aid of Gibbs ensemble Monte Carlo simulations. The predicted tie lines agreed with the experimental data within the uncertainties up to near the critical point.