Förster Resonance Energy Transfer

The MS shall be described by singly excited electronic states which may constitute Frenkel-like excitons. Those are the correct excitations if wave function overlap and thus electron exchange between different molecules are of minor effect. The related standard Hamiltonian reads [math]\label{h-fx} H_{\rm FX} = \sum_{m} E_m \ket{\phi_m}\bra{\phi_m} + \sum_{m \neq n} J_{m n} \ket{\phi_m}\bra{\phi_n} \; .[/math] It includes the molecular excitation energies (site energies) [math]E_{m}[/math] where [math]m[/math] labels the individual molecule and the EET (excitonic) coupling [math]J_{m n}[/math].