For the final reference values for the following benchmark study, the DLPNO-CCSD(T 1 )/TightPNO/CBS (3,4) values were used for the singlet REs of CDO, NiSOD and Hc, while estimated (T 1 ) was used for the triplet states of NiSOD and Hc. Co-NHase, as the largest model in the set with a 3d metal centre, required the estimated TightPNO approach. It is a closed shell cobalt enzyme with no significant multireference character, therefore we believe that the error of this level of theory will not be unreasonable for this system. The estimated CBS (3,4) approach was used for all other systems. At this point, we have cut three additional steps from the set (one each from DMML, AAP and PTE) that had reference REs of <0.5 kcal/mol, as these give unreliably high percentage deviations for the tested DFAs. The reference values for the final 55 data points in the set are given in table .