For FF-based molecular modelling to be worthwhile, the FF must be accurate. That is, it should accurately describe the potential energy surface of the target molecule, and adequately describe the vital non-bonded interactions between the molecule and its (often condensed phase) environment. In an attempt to achieve this accuracy, most transferable FFs are parametrized following a similar philosophy. Specifically, a representative subset of small molecules is selected that contains key functional groups, such as those that appear frequently in drug-like molecules . For these molecules, parameters are then fit to a combination of experimental and quantum mechanical (QM) data, and transferability between similar chemical environments is assumed.