We started with all available measurements of the properties listed above. When multiple values were available for a given quantity, only the ones with lowest reported uncertainties were retained. We furthermore excluded ionic liquids, compounds containing elements other than H, N, C, O, S, F, Cl, Br, and I, and data measured outside the temperature range 288-318K and the pressure range 0.95-1.05 atm. Dielectric constants < 10 were also excluded, because a force field that does not explicitly include electronic polarizability is not expected to replicate lower dielectric constants well. Finally, a greedy search was performed on the remaining data to select a minimal subset of small molecules that exercised the largest number of nonbonded parameter types and for which the most measurements were available. Sample compounds from the resulting set are shown in Figure , and further information on the data set can be found in Supporting Information section1.2.2.