Despite its charge-separated character, the CR state may have a small or vanishing dipole moment if c 1 ≈ -c 2 , which is often the case for high-symmetry (or even nearsymmetry) dimers. Gas-phase calculations suggest that the charge-separated states in (PDI) 2 are indeed of CR form. Here, we consider two geometries of (PDI) 2 that are taken from Ref. 56 and which afford the fastest and slowest singlet fission rates of the eight (PDI) 2 geometries that were examined in Ref. 57. The geometries of these two dimer models are illustrated on the left side of Fig. and their frontier molecular orbitals are illustrated in the middle part of Fig. . The two dimers have somewhat different offset-stacking arrangements for the cofacial PDI monomers and also different face-to-face distances. We will examine the S 1 ( 1 ππ * ) state of these dimers and also two different ME states that will be designated 1 ME (lower state) and 1 ME (upper state), following Ref. 57.