We select models out of each cost group that are close to the Pareto front for both the train and test RMSE. In the very-low-cost group M 1,2 and M 2,1 contain the same primary features and have a similar performance with train RMSE of 0.14 eV and test RMSEs of 0.15 eV and 0.14 eV, respectively. Therefore we use an ensemble of the two models: