When the chromophores are at close-contact (van der Waals) separation, there is also the possibility of intermolecular CT, which we might represent using basis states |Ψ + 1 Ψ - 2 and/or |Ψ - 1 Ψ + 2 . For highly symmetric systems, these these forward and backward CT states may be degenerate, leading to the formation charge-resonance (CR) states, which are characterized by equal amounts of forward and backward CT. Furthermore, if the electron-transfer process is similar in energy to the S 0 → S 1 monomer excitation energy then either CT excitons or else localized CT states may further mix with FE states. This type of mixing has been widely discussed in the theory of excimers and photoluminescence. These various scenarios are illustrated schematically in Fig. .