For each of the three molecules, training sets for the lower fidelity response (PBE) were chosen that included between 10% to 70% of all structures (in increments of 5%). These training sets were chosen to be nested, that is, the 65% set is a subset of the 70% set and so on. For each of these training sets, nested subsets were chosen again containing between 10% to 70% of structures (in increments of 5%) for which the higher fidelity response (PBE0) was made available. The size of these high level training sets are given as percentages of the lower level training set from which they are drawn. Each subset was chosen as a maximin space-filling design , maximizing the minimum Euclidean distance in descriptor space between structures. Hence a collection of 169 nested training sets were constructed, containing between 1% and 49% of the whole data set. We use the nomenclature "L%/H%" training set to refer to a set containing evaluations of PBE for L% of the original structures and evaluations of PBE0 for H% of the PBE training set. The 30% of structures that were not included in any training set were reserved as a test set.