A major flaw in the formalism is that the role of other states on the transition of interest ie. Herzberg-Teller components, are not accounted for. This simplification was used across numerous studies during the legacy of internal conversion theory , since the model omits some auxiliary restrictions, such as the magnitude of the electronic or vibrational energy gap, but rather focuses on the nature of the weak-coupling regime, and considers the initial state energetically well spaced from all other states as well as maintaining that the adiabatic potential surfaces are harmonic. That being said, coupling strength between two states is proportional to the energy gap, due to the nuclear kinetic energy operator inducing transitions between the two adiabatic Born-Oppenheimer states. Therefore, in order to correctly model internal conversion in any system, it is necessary to go beyond the Born-Oppenheimer approximation of quantum mechanics.