In the present study, we evaluated the dependence of the theoretical activities and linear scaling relationships of metalloporphyrin catalysts on the particular functional used. Comparisons of the relative energies of intermediates with DLPNO-CCSD(T) showed that GGAs tended to overstabilize axial oxygen-containing ligands, double-hybrids understabilized them, and single hybrids stabilized them according to the amount of exact exchange in the functional. For the present systems, B3LYP, with its 20% exchange, produced the best accuracy while also minimizing bias toward over-or under-binding.