where E m (E n ) is the energy of the m th (n th ) electronic state. This is valid until vibronic states from different electronic configurations begin to converge, to which the approximation begins to fail. Known cases of this include the Jahn-Teller effect (degenerate electronic states), the pseudo-Jahn-Teller effect (near-degenerate electronic states), and the Herzberg-Teller effect (vibrational participation in electronic transitions). Following, a standard treatment of first order perturbation theory is performed, leading to an expression for the probability Ω n for locating the system in state |ψ n ⟩ :