Of the low-cost group, M 4,1 and M 2,2 yield the same RMSE of 0.14 for the training set. Of the two, M 2,2 delivers a better performance for the validation set with an RMSE of 0.18 eV without terphenyl (0.27 eV with terphenyl), compared to 0.20 eV for M 4,1 (0.27 eV with terphenyl). Therefore, we select M 2,2 :