The one-body term is only meaningful if the atoms are in an external field (e.g. an electric field). In the absence of external fields, the potential V should not depend on the absolute position of atoms, but only on the relative positions. This means that the functional form can be rewritten as a function of interatomic distances r i j = | r → → i − − r → → j | and angles between the bonds (vectors to neighbours) θ θ i j k . Then, in the absence of external forces, the general form becomes