Computational studies aid the design of novel catalysts in part by defining the factors that limit theoretical catalyst activity. Density functional theory has shown that most ORR catalysts are constrained by linear free energy scaling relationships (LFESRs) between the binding energies of reaction intermediates. These scaling relationships can be used to derive characteristic volcano plots that relate catalyst activities to the binding energy of a single ORR intermediate. Such plots show that Pt is near, but not at, the theoretical peak activity of metallic surfaces, and that a significant overpotential will remain unless these LFESRs can be broken or circumvented by a novel class of catalyst. As a result, new catalytic materials are sought to push catalyst activity past typical volcano plot scaling and provide alternatives to Pt. Porphyrins and M-N-C sites more generally have attracted considerable attention as non-platinum group ORR catalysts with high theoretical limiting potentials and experimentally demonstrated activities competitive with Pt-based catalysts. Because of the tunability of functional groups on these porphyrins, their activity and selectivity toward either the two-electron or four-electron ORR can also be optimized, making them promising scaffolds on which to build next generation ORR catalysts. A cycle depicting possible mechanisms for both of these pathways on a metal porphyrin Despite their promise, describing the electronic structure of transition metal porphyrins is no trivial endeavor. These molecules possess complicated electronic structures which make the determination of ground spin states and binding energies challenging and highly dependent on the particular density functional approximation employed. Studies aimed at related molecules have indicated that single hybrid functionals with small amounts of Hartree-Fock exchange perform well relative to wavefunction theories and experiment. The recommendation of these functionals also agrees with generalized benchmarking efforts on transition metal complexes (an overview of these works is provided in Table ).