where each term t is a simple product of one-mode operators h m,t with an associated coefficient c t . Using this representation of the PES, the Hamiltonian in Eq. 1 can be written in a sum-over-products form. The computational advantage of this representation is that instead of performing multi-dimensional integrals and large contractions, we do an intermediate fit to obtain a sum-over-products form with one-dimensional integrals and simple contractions.