Importance of density functional approximation for the prediction of metalloporphyrin catalyst activities for oxygen reduction

Metalloporphyrins, a class of compounds widely encountered in nature, present intriguing structural and electronic properties for a number of applications, including as oxygen reduction catalysts. However, computational investigations of metalloporphyrin-catalyzed reactions have been hampered by their complex electronic structures. In the present study, we test the sensitivity of metalloporphyrin catalyst activities to the chosen functional in density functional theory. We find a significant split between pure (GGA) and hybrid functionals, with hybrid functionals, in particular B3LYP, showing greater agreement with DLPNO-CCSD(T) reaction energies. Notably, double-hybrids offered no noticeable improvement over the much more computationally efficient B3LYP and PBE0. Other discrepancies between functionals, including ground state spin and geometry, are also considered in this work. Finally, both hybrid and double-hybrid functionals greatly reduced the gas phase errors associated with the main group molecules in the oxygen reduction reaction relative to GGA calculations, leading us to question the application of widely used empirical corrections to O$_2$.