We study the isomerization reaction coordinate (RC) in the M(IV)=O intermediate for both Fe and V, and compare their energy landscapes. When choosing a starting point for this isomerization, we note that while the bidentate Glu isomer is slightly more stable than the monodentate Glu isomer, we study active site isomerization for isomers where succinate or Glu or both are monodentate. This modification allows for more flexibility in the active site to enable isomerization of the reacting moiety, i.e., oxo/hydroxo. Importantly, we found small energetic differences between monodentate and bidentate succinate/Glu isomers for the Fe(IV)=O intermediate, i.e., ca. 3 kcal/mol (Figure ).