We did not include charged molecules in the current training or test sets, because our use of QM gas-phase calculations as training data does not account for any differences in properties between gas phase and in aqueous solvent, which would be more pronounced in charged species. Therefore, the accuracy of this force field for charged species is presently unknown. This issue has been addressed in recent work on protein force fields in which the QM training data and MM calculations are both carried out in the presence of implicit solvent models, which may also be applicable to small molecule force fields. More comprehensive benchmarks are planned to inform future force field generations. However, the present level of theory is superior to the HF/6-31G* approach used in parameterizing the parm99/parm@Frosst force fields from which SMIRNOFF99Frosst descended, and thus should afford greater accuracy.