Thus, the evaluation of the force on the λ-particle requires that the potential energy, including the long-range electrostatic interactions, is computed twice: once for λ = 0 (i.e., V A ) and once more for λ = 1 (i.e. V B ). If the Particle-Mesh-Ewald (PME) method is used to compute those long-range electrostatic interactions, separate PME grid builds are needed, because the charge distributions are not identical in states A and B.