describes a system of interacting electrons and nuclei, whose positions are collectively indicated as r and R, respectively. The nuclear kinetic energy operator is expressed in Cartesian coordinates and contains a sum over the N n nuclei, each labeled with the index ν, with spatial derivatives ∇ ν with respect to nuclear positions; M ν are the nuclear masses. The electronic Hamiltonian Ĥel (r, R) is the sum of the electronic kinetic energy and of all interactions. The time evolution of the electron-nuclear system is dictated by the timedependent Schrödinger equation (TDSE)