We also obtain isomerization RCs in which we swap one metal center into the geometry of the alternate metal, i.e., Fe(IV)=O as single-point calculations of the optimized geometries along the isomerization RC of V(IV)=O and vice versa (Supporting Information Figure ). These frozen RCs are similar to the relaxed RCs for both Fe and V, except for the small increase in energetics owing to the fully constrained geometries, suggesting that geometric structure plays only a relatively minor role relative to electronic structure and spin state differences of Fe and V play in isomer energetics (Supporting Information Figure ). Overall, the isomerization RCs connecting axial oxo and equatorial oxo isomers differ significantly between Fe and V, indicating than 1 kcal/mol (Supporting Information Figure ). The energetic barriers to move from one minimum to another are also relatively minor, i.e., ca. 1 kcal/mol (Supporting Information Figure ). Overall, given the more favorable energetics of RCOH1 relative to RCOH2, we can expect that hydroxo isomerization would be accompanied by hydrogen bonding interactions between hydroxo and Glu, although the presence of the hydrogen bond does modestly increase isomerization barriers (Supporting Information Figure ).