Marcus theory (Equation ) is valid when any nuclear changes occurring only involve modes with smaller energies than the molecular thermal energy within the classical limit . As such, there are plenty of cases where the use of Marcus theory yield results that are sceptical at best. Zhang and co-workers examined PM6 in a number of donor-acceptor configurations and differing geometries, but universally found internal conversion rate constants of < 1 s -1 . They argues that this behaviour corresponds to a bi-molecular decay at the heterojunction interface, however even considering this as a possibility, the result is still too impossibly small. It is much more likely that this system is not viable for modelling through Marcus theory; internal conversion may be dominated by Herzberg-Teller contributions, as there is no consideration for vibrational effects.