Ideally we would additionally test these estimated levels of theory against full DLPNO-CCSD(T 1 )/TightPNO/CBS (3,4) for some open shell systems as well, but the open shell implementation of DLPNO-CCSD(T 1 ) is significantly more costly, so quadruple-ζ calculations with (T 1 ) triples were unfeasible. The models of Hc and NiSOD are small enough that the energies of the triplet models can be calculated at the estimated (T 1 ) level of theory, so we have used these for a brief assessment of estimated CBS (3,4) on open shell systems (table of the SI). The MAD of estimated CBS (3,4) against estimated (T 1 ) across this small set is 0.336 kcal/mol, although we note that this is likely underestimated as the equivalent MAD est.(T1) across the closed shell models tested here is 0.093 kcal/mol, significantly smaller than the MAD against DLPNO-CCSD(T 1 )/CBS seen in table 3 (0.234 kcal/mol).