While changing the amount of global Fock exchange does not improve the results, introducing range separation does-CAM-B3LYP is noticeably better than B3LYP, and the same is seen for the double hybrids (ω)B2PLYP and (ω)B2GP-PLYP. Another feature that leads to better functional performance is spin-component- or spin-opposite-scaling (SCS/SOS), where different amounts of the same spin and opposite spin contributions to the MP2 energy are included. Many previous studies have shown that SCS-and SOS-MP2 are better than standard MP2 for transition metals, and here the SOS DFAs are particularly strong. SOS0-PBE0-2-D3(BJ) and PWPB95-D3(BJ) are first and fifth among the double hybrids, and revDOD-PBEP86-D4 (fourth best) performs slightly better than its SCS equivalent revDSD-PBEP86-D4. Finally, we look at the choice of basis set, as large transition metal complexes and open shell systems can be quite computationally demanding, so faster methods that maintain accuracy are desirable for computational studies of metalloenzymes. We do not recommend cutting costs by using GGA or meta-GGA/NGA functionals, as the results in fig. and tab. 5 show that these are significantly less accurate. We specifically note that MN15-L, the best third-rung functional on MADs, was the slowest converging functional overall for most structures, so it provided no cost benefit over a good hybrid or even double-hybrid. However this may be due to the LibXC 155 implementation evoked by ORCA, as MN15 and MPW1B95 were also slow. We also cannot recommend the -3c composite methods, as although they are fast, they perform poorly-PBEh-3c is the worst hybrid, while r2SCAN-3c and B97-3c are both the second worst in their respective rungs. We therefore test smaller basis sets for each of B3LYP-D3(BJ), M06-D4, PWPB95-D3(BJ) and ωB2PLYP-D4 (the best dispersion combination for each functional) in fig. . Between def2-SVP and def2-TZVPP there is a significant reduction in the MAD, error range and distribution of the data for all four DFAs, confirming that double-ζ basis sets are not adequate for calculating BHs and REs. Between def2-TZVPP and def2-QZVPP, however, the results are almost identical. All MADs are the same, and the error ranges only reduce slightly with the larger basis set (1.5 kcal/mol for ωB2PLYP-D4, and less than 1 kcal/mol for the other DFAs). PWPB95 in particular is known to have a weaker basis set dependence than other double hybrids, due to its relatively low fraction of SOS-MP2 correlation. 150 Thus, we can say that the DFT energies are adequately converged at the triple-ζ level, and it is not worth the extra cost to go up to a quadruple-ζ basis set for calculating REs and BHs in such systems.