With this in mind, we next consider an example relevant to organic photovoltaics, namely, a pentacene dimer model of a chromophore that undergoes intramolecular singlet fission, which is depicted in Fig. . Calculations are reported here using the unlinked dimer model that is shown in Fig. ). We have confirmed, via CIS calculations, that the model system and the full chromophore afford excitation energies within about 0.3 eV of one another, which is sufficient for our purposes. The same is true for two other truncated models (not shown) in which the side chains or the spacer group were removed, but not both, indicating that the tri-isobutyl silyl side chains in the full chromophore [Fig. ] function mainly for solubility while the spacer moiety functions mostly to position the pentacene chromophores relative to one another. The singlet fission dynamics can be quite sensitive to the relative position and orientation of the pentacene moieties, 67 so the atomic positions of the pentacene dimer core are the same in the model system as they are in the full chromophore. Calculations on the model are reported below in six different solvents, and in each case we compute six RAS-SF states: the ground state, the first locally-excited state (called LE2 here because LE1 corresponds to the ground state, consistent [Eq. ( )].