A second choice related to the physical description of the force-field is the atom types used. Atom types are usually kept as general as possible but a common distinction made is to treat hydrogen atoms connected to a polarising element such O or N with different parameter values . To avoid overfitting, we treat hydrogen atoms bonded to O or N atoms equivalently and denote the atom type as Hp (hydrogen-polarised), whereas hydrogen atoms bonded to carbon or sulphur are denoted Hc .