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, Deltamu(w)(0) = 1077 +/- 5 kcal/mol and DeltaH(w)(0) = 1294 +/- 11 kcal/mol, were developed for Structure 11 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, Deltamu(w)(0) = 1203 +/- 3 kcal/mol and DeltaH(w)(0) = 1170 +/- 19 kcal/mol, for structure I hydrates using the van der Waals and Platteeuw model were also determined. Equilibrium predictions with an average absolute deviation of 3.4% for the mixed hydrate of argon and methane were made. These accurate predictions of the mixed hydrate system provide an independent test of the accuracy of the intermolecular potentials. Finally, for the mixed argon-methane hydrate, conditions for structural changes from the structure I hydrate of methane to the structure 11 hydrate of argon were predicted and await experimental confirmation.