To ensure that reliable single reference benchmark energies could be calculated for the systems, the multireference character of each structure was checked with a range of diagnostics. First A λ values 173 were obtained using the PBE and PBE0 density functionals. Then DLPNO-CCSD(T 0 )/NormalPNO/def2-SVP calculations were done to test the %TAE(T), 174 T1 diagnostic 175 and largest T 2 amplitude. The A λ and %TAE(T) approaches test the sensitivity of the total atomization energies to the level of theory (namely Hartree-Fock and higher order coupled cluster excitations, respectively). A λ ≤ 0.1 and %TAE(T) ≤ 2% generally mean that a system is dominated by dynamic correlation, while values of A λ ≈ 0.15 and %TAE(T) up to 5% indicate mild multireference character. For the T1 diagnostic, typically a threshold of 0.02 is used, but Wilson and coworkers state that this can be increased up to 0.05 for systems containing transition metals. While thresholds for the largest T 2 amplitude are not as clearly defined, generally values of 0.1-0.2 have been considered mild but not necessarily problematic when other diagnostics are low, while some say up to 0.15 can be considered low. The results of these diagnostic tests are summarized in table 2. %TAE(T) values could not be calculated for Mo-Cu CODH due to convergence issues for the Mo atom calculation, but given that no systems have %TAE(T) ≥ 2%, even those which are indicated to have mild multireference character by the other diagnostics, it is likely that these values would also be low. Looking at the other diagnostics, we see that Table : Summarized results of tests of the multireference character of each system, with values indicating mild or moderate multireference character shown in bold and the number of structures above the threshold for each diagnostic given in parentheses. Overall ranges for each reaction are given, while full results can be found in the Supporting Information. Any reaction steps involving structures that are flagged by three or more diagnostics are cut from the final set. The DLPNO-CCSD(T) calculation could not be converged for the Mo atom or NiSOD structures 3 and TS5, so some diagnostics could not be obtained. These two NiSOD structures were, thus, also cut from the set. none of the T1 values are greater than the revised 0.05 threshold for transition metals, and even the largest A λ values still only indicate mild multireference character. The trends across the diagnostics, however, are a more reliable indicator of problematic behavior than any individual one. We therefore use the more conservative T1 = 0.02 and largest T 2 amplitude = 0.1 thresholds, along with A λ = 0.1, and any structures that are above the thresholds on all three diagnostics are removed from the set.