This is a remarkably simple expression which relates the change in optimal intermolecular energy to the derivative of the repulsive/dispersive energy contribution evaluated at the optimal crystal geometry. This is despite the additional indirect dependence of U inter,s on the parameter values through Ω * s (p, Ω † s ). The required derivatives of the energy residual, ∂E s /∂ pk in Equation , can be shown to be directly proportional to this derivative.