Predictions and true values of the PBE0 lattice energies for the oxalic acid test set (30% of original structures) are presented in Figure for three modelling strategies. Model A (Fig. ) is the ARGP model, trained on PBE evaluations for 30% of all structures (training set I) and PBE0 evaluations on 30% of this set (9% of all structures, training set II). These are compared to predictions from two single-level PBE0 GPR models: model B (Fig. ) is trained on PBE0 energies obtained for training set I; and model C is trained on the PBE0 energies for training set II (Fig. ). Hence, model B had available a greater number of expensive training points than was the case for model A, and model C had the same number of expensive training points as model A but did not make use of the larger number of cheaper points.