Calculations on (PDI) 2 in two different geometries provide a rationale to understand the very different singlet fission rates that have previously been calculated for these systems. These calculations suggest that dielectric stabilization creates excitonic trap states in one geometry but not the other, by breaking symmetry and localizing charge. This observation has potentially important implications for the design of materials with good singlet fission rates, as the excitation spectra of model chromophores can now be easily tested as a function of solvent polarity. It also suggests that gas-phase calculations, even with highly correlated wave function models, may not offer a realistic description of singlet fission in solution, where polarization-induced charge localization is observed in solvents with dielectric constants as small as ε 0 = 3, but not in vacuum (ε 0 = 1).