The M 3,3 and M 4,3 models, whose computational cost is considerably higher, have a better accuracy for the training set. However, their RMSE increases significantly for the unseen test set, which is indicative of over-fitting. This is also seen in the correlation plots in Figure ,c. Interestingly, SISSO does not produce any models that can predict the crystal fundamental gap based only on single molecule features (the equations of all models are provided in the SI).