This way, normal modes can be categorised into one of two paradigms : either a mode with some non-zero coupling between some initial zeroth-vibrational ground state and some final n th -vibrational excited state, or a mode displays zero coupling, and acts as an energy reservoir and inhibits quantumnumber reversibility (opposite-direction transition via identical vibrational quantum numbers). Importantly, the former participates in the transition and the latter does not. The coupling between initial and final electronic states, when the initial state is fixed at the zeroth-vibrational level ⟨ψ ns |H| ψ m0 ⟩ can be found using the one-electron nonadiabaticity operator , cast using internal coordinates Qj within the Franck-Condon approximation as :