A natural question is whether the level of theory used for geometry optimization really matters energetically, for instance when computing catalyst activities. Figure (b) quantifies the energetic difference between each geometry optimized at the BP86 level and the energy of the re-optimized geometry for each other functional we tested. These relative energies are plotted against the RMSD for BP86 and re-optimized geometries as described in Figure (a) to understand how sensitive these systems are to small structural changes. For the most part, the structural and energetic differences remain small (< 0.1 Å and < 5 kcal/mol, respectively) across all intermediates for a given functional. This indicates that the relative energies of intermediates, and therefore computed catalyst activities, are only minimally affected by the functional used for geometry optimization. However, there are some notable exceptions. The weak-binding intermediates FeP(O 2 ), CoP(O 2 ), and NiP(OOH) all show intermediate to large RMSDs and large energy differences for at least one functional. These results suggest that caution should be taken when determining the structures of weak binding MP intermediates, though we note that this is rarely an issue for the determination of overpotentials since the dioxygen intermediate is not required for these.