We perform a semi-empirical separation of the movements of cores and valence electrons in a molecule, generating coupled equations for the movement of the cores. Far from critical points of the PES, the adiabatic correction depends just on the valence wavefunction. Nonadiabatic effects turn dependent on the valence wavefunctions as well and can be evaluated by replacing the nuclear reduced mass by another one lying between the nuclear and the atomic reduced masses. A formula for the effective core mass based on electron atomic populations is introduced. Nonadiabatic corrections to vibrational levels of LiH are reported.