First principles models of the interactions of methane and carbon dioxide

We present ab initio calculations on the CH4–CH4 and CH4–CO2 dimer potential energy surfaces. We show that the fit to these surfaces is improved over the Lennard–Jones potential by changing the form of the repulsive wall. Ab initio calculations are also performed on trimers to evaluate the strength of nonadditive interactions. The experimental CH4 phase properties are reproduced in simulations that include correction for nonadditive dispersion. The phase-coexistence curve of mixtures of CH4 and CO2 is also improved by including this nonadditive dispersion term.