The CRPS from the ARGP model A is 1.114 kJ mol -1 , compared with 1.112 kJ mol -1 for the predictions obtained from the expensive single GP model B. To obtain this very minor improvement requires an increase in computational cost of more than a factor of 3: just 9% of the expensive PBE0 data used for training model A compared to 30% for model B. The PBE calculations needed for the ARGP model have only a small influence on the total computational cost. Model C, on the other hand, has a comparable cost to the ARGP model; the difference in computational costs occurs only from using the PBE data for 30% of the structures in model A. The CRPS for this single GPR model is considerably higher, at 3.96 kJ mol -1 .