The mean absolute errors and standard deviations of errors given in Tables and are biased estimates. The training set data was compared to the model predictions in each case. The polymer FMO energies of a validation set containing 10 compounds were compared to the values predicted by the polymer models in Table to give a more accurate picture of the errors associated with these models. The results of this comparison are given in Table . The non-biased cross-validation sets have higher errors than the training sets, but these errors are still relatively small (e.g. 0.26 eV MAE for the polymer Eg). This suggests some overfitting may occur, as expected due to the relatively modest training set size of 60 compounds.