Hydration and solvation free energies have proved particularly valuable in quantitatively testing all-atom molecular simulations and force fields and in highlighting systematic errors, in part because ∆G solv and ∆G hyd for small molecules can be calculated to a precision better than 0.1 kcal/mol. The SAMPL series of blind challenges for computation have helped illustrate this, with several challenges focusing on solvation and hydration free energy prediction. SAMPL1 through SAMPL4 featured blind prediction of hydration free energies. SAMPL5 included the prediction of partition coefficients, which also required the calculation of solvation free energies in water, cyclohexane, and octanol. Likewise, the FreeSolv database of calculated and experimental hydration free energies has been broadly useful for similar reasons. Work on FreeSolv has helped highlight and resolve various force field problems, such as with hydroxyl group parameters. Even though there are experimental and calculated hydration free energy databases such as FreeSolv and ATB, ∆G solv and ∆G hyd measurements are difficult and require considerable expertise and specialized equipment, so few to no experimental measurements are presently made. Infinite dilution activity coefficients, however, are a critical property of consideration when studying binary mixtures, and even more so as the number of components and/or the system size grows for industrial applications, 3 meaning that they are subject to considerable experimental attention. Additionally, they are easier to measure than ∆G solv and ∆G hyd , and can be calculated with similar precision as the aforementioned free energies. This means they are an ideal candidate as an alternative to hydration free energies for benchmarking computational chemistry methods and force fields.