First principles models of the interactions of methane and carbon dioxide

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We present ab initio calculations on the CH 4–CH 4 and CH 4–CO 2 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 CH 4 phase properties are reproduced in simulations that include correction for nonadditive dispersion. The phase-coexistence curve of mixtures of CH 4 and CO 2 is also improved by including this nonadditive dispersion term.