where N is the number of interacting atoms contained in a volume V , k B is the Boltzmann constant, T is the temperature, r i represents the position of atom i, and f i is the total force acting on atom i. The term enclosed in the angled bracket on the right of the equation is the internal virial, which arises from forces acting between atoms in a system. However, as pointed out by Louwerse and Baerends, this equation is not applicable in PBC. For systems with pair-additive force fields, pressure can be expressed as