For most of the other functionals, the deterioration in performance is far from uniform, and at times spectacular. For example, M06, PBE0, and B2-PLYP, which appeared fairly robust when subdividing the Diels-Alder reactions of BH9 into two benchmarking subsets (cf. Figure ), exhibit a mean error at least three times larger when evaluated on our active learning dataset instead of on BH9. In absolute terms, the deterioration of BLYP, CAM-B3LYP and BHandHLYP is even worse, but as indicated before, adding dispersion corrections remedies the situation to some extent (cf. Section S3 of the ESI). The ωB97M-V and M06-2X functionals on the other hand remain relatively robust, though the MAEs still deteriorate by approximately 2 kcal/mol. Because of this deterioration, the former functional, which performed best on the original BH9 subset, now only comes in fourth place when ranking the functionals based on performance.