The ORR catalytic activity of metalloporphyrins was assessed for the 4-electron associative pathway depicted in Figure and in the reaction scheme in eqs 1(a)-(e). The reaction proceeds through the intermediate complexes MP(O 2 ), MP(OOH), MP(O), MP(OH). It is worth noting that many ORR studies neglect the initial step of this pathway, the binding of O 2 to the catalyst. However, we have included this step because 1) it shows a large dependence on functional choice, 2) there is ample experimental and computational data on this step available for comparison, and 3) the oxygen binding geometry and energetics are correlated with molecular catalyst activity for the ORR. As the goal of this work is to assess the variability and accuracy of various functionals, the porphyrins themselves were kept unfunctionalized. This minimized the computational complexity and facilitated the use of high accuracy single-reference methods.