In Table , the atomization energy of O 2 shows the expected large error for GGA functionals but a significantly reduced error for hybrid functionals and B2PLYP. For these higher-level functionals, these errors are of similar magnitude to those in the H 2 and H 2 O atomization energies, leading us to believe that hybrid functionals treat all of these species with a comparable level of accuracy. Some caution is advised when using the accuracy of atomization energies to draw conclusions about a functional's accuracy for thermochemistry, as these do not correlate well in general. For GGA functionals specifically, errors in atomization energies often derive more from a poor treatment of the spin polarized single atoms than from the treatment of molecules. Bearing this in mind, we still believe that the uniformly small errors in atomization energies across all species for (double-) hybrid functionals indicates that the error associated with the O 2 molecule can no longer be considered a priori to be an order of magnitude worse than H 2 O and H 2 . This assertion is supported by recent work on H x N y O z molecules, which concluded that unlike with GGA functionals, corrections for O 2 were were unnecessary to produce accurate formation enthalpies when using B3LYP and PBE0. It thus seems likely that for (double-) hybrid functionals, residual errors in ∆G 0 ORR have more to do with the incomplete cancellation of relatively small errors spread across all three species in the reaction than with a single error concentrated on O 2 . Atomization energy errors in Table suggest that lack of error cancellation is particularly to blame for the poor reaction free energy of TPSSh, while the relatively good free energy of PBE0 results from some fortuitous cancellation. If this is the case, merely shifting the O 2 free energy to correct the overall reaction free energy means that this O 2 energy absorbs the errors associated with H 2 and H 2 O, introducing a bias that depends on the distribution of these errors, which is not systematic across functionals (Table ).