thus only one electronic state φ BO (r; R), e.g., the ground state, is considered in the dynamics. On the other hand, the exact factorization yields the molecular wavefunction as in Eq. ( ), i.e., Ψ(r, R,t) = Φ(r,t; R)χ(R,t), which is formally similar to Eq. ( ), as it is a single product of a nuclear wavefunction and of an electronic wavefunction with a parametric dependence on R. In the BO approximation, one usually introduces a static potential energy surface and static vector potential, or derivative coupling, namely