Breaking down the differences between functionals, we found that ground state spin state predictions were quite dependent on the presence of exact exchange in a functional and could have a significant impact on theoretical catalyst activities if not properly assigned. Pooling the predictions of functionals with different amounts of exchange, however, almost always generated an assignment in line with experiment and DLPNO-CCSD(T). On the other hand, geometries were generally resilient to the choice of functional. Exceptions included the metal-oxygen bonds of weakly bound intermediates and some striking difficulties with geometry convergence and poor energies for spin-contaminated wavefunctions when using B2PLYP, which highlight the potential pitfalls of using double-hybrid functionals for these systems. Finally, we believe the commonly used empirical O 2 correction could be an additional source of variation among functionals due to the different accuracy of GGAs and hybrids in treating O 2 . This correction should be reassessed as hybrid functionals become more commonplace in theoretical catalysis studies.