It is worth noting that Marcus' model has been known to fail for systems that fall within the so called "inverted region" ; as E 0 λ becomes more negative the rate slows down, opposing the exergonicity. It is also worth highlighting that the ionisation potential and electron affinity can be approximated as the highest occupied and lowest unoccupied molecular orbitals, respectively ; whereby the generalised Mulliken-Hush model as the coupling is not valid for many systems. A good example of this was shown by Biswas and co-workers where the rate of internal conversion was less than <1 s -1 for a series of fused heteroacenes.