with the non-relativistic electron-nuclear Hamiltonian Ĥ(r, R), and whose solution is the time-dependent molecular wavefunction Ψ(r, R, t). Electronic and nuclear coordinates are collectively indicated with the symbols r and R, respectively. The Hamiltonian is the sum of a nuclear kinetic energy operator Tn (R) = Nn ν=1 (-2 ∇ 2 )/(2M ν ) -in Cartesian coordinates -for the N n nuclei with masses M ν , and of the electronic Hamiltonian Ĥel (r, R), which is, in turn, the sum of the electronic kinetic energy and all interactions.