The overall objective function for the test set is lower for Parsley (20,672) than for the initial force field (29,469). The distribution of improvements over the test-set compounds (Figure ) shows that the objective function improves for almost all compounds, given that the blue histogram of differences has few positive values. Accordingly, improvements of 6-35% are observed in the terms that contribute to the objective function (Table . It is worth noting that the test set exercises 415 out of the 500 parameters. We also grouped the bond lengths, bond angles, and improper dihedrals across test set compound according to their FF types and examined the improvement in accuracy by type, as illustrated for the bond-lengths in We also tested the ability of the Parsley force field to replicate differences among conformations of gas-phase molecules in the test set. Note that this type of data is entirely absent from the training data.