The FIT parameterization distinguishes atom types based solely on the corresponding chemical element, with the exception of hydrogen atoms which were allowed to adopt different parameter values if bonded to a polarising element such as oxygen or nitrogen. Crucially, only homoatomic interaction parameters were explicitly parameterized while cross-interactions between heteroatomic atom types were calculated using predetermined combining rules. For a set of reference crystal structures, parameters were then optimised so as to minimise the forces exerted on each atom at the experimental crystal geometry, as well as the difference between computed lattice energies and measured sublimation energies. A flaw of the empirical force-fields derived using this approach is that when they are used within an energy minimisation, the resulting crystal geometry may no longer be a close match with the reference geometry. The parameterization proposed by Williams was derived using a similar methodology, but atom types were further distinguished to account for bonded-neighbours and the local molecular environment of each site. This more elaborate atom typing scheme was applied to both heavy atoms in addition to hydrogen atoms.