with φ k (r; q, R) the set of eigenstates of the Hamiltonian VPEN (q, R) = ĤPEN (r, q, R) -TN (R) -TP (q), with eigenvalues E k (q, R), representing the electronic PESs as functions of the photonic and nuclear coordinates. The nonadiabatic couplings between two of these electronic states encode the information on the variation along R (∇ α ) or along q (∇ ν ) of the states; they will be indicated collectively as