where φ n (q, Q) and χ n (Q) are the electronic and nuclear wavefunctions, respectively. The eigenstates of Equation 3 are not stationary, but rather the whole system oscillates between various quantum states ; that this what we interpret as the electronic transition, followed by a transition in the quantum states of nuclear motion. Therefore, to properly probe internal conversion, the perturbation resulting in the transition between electronic states needs to be modelled correctly. At the time, a method to calculate an explicit perturbation term was unavailable. However, the transition probabilities can still be obtained as highlighted by Kronig .