The first, termed the Roche Set, contains 468 fragment-sized molecules, most containing one to three rotatable bonds, that were provided by Roche as a collection of important and/or interesting chemistries. This data set was prepared using the MOE software. Representative compounds from this set are illustrated in Figure , and the full set can be found in Supporting Information section 1.1.2. The second, termed the Coverage Set, contains 80 molecules selected from eMolecules 46 using a greedy algorithm aimed at providing parameter coverage for the maximum number of parameters using the minimum number of molecules. Initial automated selection of the Coverage Set is described in a subdirectory of the openforcefields GitHub repository, and details of the additional molecules added manually to cover remaining gaps can be found in Supporting Information section 1.1.2.