For the representation of the oligomers in the ML model we began with the monomer ID and the oligomer length. In addition, we added the average dihedral angle between the monomers of each oligomer and the average inter-ring bond length of each oligomer, for both the neutral and cation species of each oligomer, as calculated using GFN2. We saw correlation (e.g., R 2 between 0.57 and 0.74) between those geometric values calculated with GFN2 and with B3LYP (Figure ). Using those starting features gave us decent preliminary results. Next, we added an extended circular finger print (ECFP4) 2048 bit representation using RDKit 36 , which increased the R 2 and decreased the RMSE of the model significantly, likely by describing local functional group effects on reorganization energies. The final step was to add a new feature to represent the size of the π-system in the oligomers (Figure ), as we hypothesized that a highly-conjugated oligomer will contribute to a lower λ. Adding this final feature moderately improved the model (Table ).