High level ab initio quantum mechanical calculations were used to determine the intermolecular potential energy surface between argon and water, corrected for many-body interactions, to predict monovariant and invariant phase equilibria for the argon hydrate and mixed methane−argon hydrate systems. A consistent set of reference parameters for the van der Waals and Platteeuw model, = 1077 ± 5 kcal/mol and = 1294 ± 11 kcal/mol, were developed for Structure II hydrates and are not dependent on any fitted parameters. Our previous methane−water ab initio energy surface has been recast onto a site−site potential model that predicts guest occupancy experiments with improved accuracy compared to previous studies. This methane−water potential is verified via ab initio many-body calculations and thus should be generally applicable to dense methane−water systems. New reference parameters, = 1203 ± 3 kcal/mol and = 1170 ± 19 kcal/mol, for structure I hydrates using the van der Waals and Platteeuw model were also dete...