and NiP(OH) despite these intermediates having small RMSDs. All of these complexes showed It is well established that the Møller-Plesset perturbation series converges more slowly for spincontaminated references and that MP2 fails dramatically in frequency calculations for geometries where spin contamination changes rapidly as a function of nuclear coordinates. However, other work has argued that double-hybrids benefit from a cancellation of the energetic effects of spin contamination at the Hartree-Fock and MP2 levels. Our results, including comparison with resricted open-shell calculations (Figure ) suggest that the studied metalloporphyrins do not always benefit from this cancellation of errors, perhaps because of the accessibility of multiple lowlying spin states that can contaminate the wavefunction. Furthermore, recent investigations have shown that even absent spin contamination, double-hybrids may be unreliable for ligands capable of both σ -donation and π backbonding. Given these drawbacks and the lack of notable improvement over single hybrid functionals, we recommend against the use of double-hybrid functionals for metalloporphyrin and related systems without careful comparison against reference data. Moving from lower to higher runs of Jacob's ladder (top to bottom in Table ) generally improves the predicted free energy of the ORR (the poor performance of TPSSh is a notable exception to this trend). Nevertheless, none of the tested functionals approach chemical accuracy (error ≤ 1 kcal/mol). Thus, semi-empirical corrections are important for the ORR. The poor treatment of O 2 atomization energies and adsorption energies relative to H 2 or H 2 O by GGA functionals is typically used to justify an empirical correction to O 2 alone in order to bring calculated G 0 ORR values in line with experiment. However, the errors associated with H 2 and H 2 O have been shown to be important for some GGA functionals. Another recent study of the 2-electron ORR showed that for both GGA and hybrid functionals, O 2 -only corrections were insufficient in the face of similar order-of-magnitude errors in gas phase H 2 O 2 . As a result, it is important to consider any similar magnitude gas-phase errors before applying an O 2 -only correction.