In this section, we consider the ptSS approach for vertical excitation energies. We first examine the model systems C 2 H 4 • • • C 2 F 4 and (naphthalene) 2 that were considered above, but we also investigate some pentacene dimers of interest in organic photovoltaic applications. These include a model system extracted from an intramolecular singlet fission chromophore, and we note that the role of CT states in the singlet fission mechanism has been much debated.