In the low-cost group M 2,2 yields the lowest RMSE of 0.12 eV for the training set, however its performance deteriorates significantly for the validation set with an RMSE of 0.14 eV. Next, M 1,3 and M 3,1 have a similar performance with a train RMSE of 0.12 eV and test RMSEs of 0.13 eV and 0.12 eV, respectively. The two models contain the same primary features, except for the single molecule ionization potential, IP S , which only appears in M 3,1 . Because evaluating IP S only requires one additional calculation of a single molecule cation, the cost of the ensemble model is only slightly higher: