where E 0 λ is the Gibb's free energy of the system. A number of investigations have reported couplings calculated using the generalised Mulliken-Hush model , while other works employ the fragment methods (excitation or spin) . These methods are typically used to treat charge or energy transfer problems rather than internal conversion; however it has been shown that they can be used to calculate internal conversion under certain conditions ; specifically, when the electron-hole overlap is small and the exciton radius is large.