where the V (q) potential is the sum of all one-mode uncoupled potentials in the system, while the V (q) potential includes also the pairwise coupling of the modes. For the higher order potentials V (k) (q) up to k modes are coupled. Each potential to a given order k consist of potential energy terms coupling a subset of the M modes in the system, where k coordinates are different from the reference value which we here set to zero