It should be clearly noted that use of a perturbative method within an harmonic paradigm, like that of the ShaMan model, can lead to displaced rate constants as larger amplitudes between oscillations can lead to larger deviations from the unperturbed Hamiltonian, such as is the case of large energy gaps. While these displacements can be minimised through cohesive coupling between vibrational degrees of freedom and the electronic states , in the case where coupling between states is very large Equation will break down, and only be able to estimate the speed of ultra-fast rate constants . If we recall the conditions in which the Plotnikov-Robinson-Bixon-Jortner model (Equation ) is valid, it becomes apparent that any rate constant faster than ∼ 10 14 s -1 should be interpreted tenuously, and in context to photophysical characteristics of the system's initial and final states.