defines a system of interacting electrons and nuclei, whose positions are collectively indicated with the symbols r and R, respectively. We indicate with T (R) the nuclear kinetic energy and with ĤBO (r, R) the sum of remaining terms, i.e., the electronic kinetic energy and all the interactions; ĤBO (r, R) is usually referred to as Born-Oppenheimer (BO) Hamiltonian. The time-dependent Schrödinger equation (TDSE) with Ĥ(r, R) dictates the evolution of the molecular wavefunction Ψ(r, R, t) as