and m = 8, that equals 1 at 0 Å, and rapidly decays to 0 when the argument exceeds 0.5 Å. An H-bond is therefore counted if the product of the two functions exceeds 0.5, and not otherwise. The presence or absence of an H-bond is facilitated because the product of these analytical functions is predominantly either close to 0 or to 1. In practice, it is defined whether H is covalently bound to either molecule i or j in order to ensure the correct counting with periodic boundary conditions. We have observed, like others 185 , that this counting is qualitatively comparable to conventional criteria that involve both the O i O j distance and the angle between the O i O j and O i H directions 136 .