Reaction coordinates (RCs) for isomerization of axial to equatorial (i.e., of oxo/hydroxo/halide ligands) were obtained using the protocol from prior work at the PBE0-D3/def2-TZVP level of theory. These RCs were sampled by rotating the oxo/hydroxo/halide ligand with respect to the axial His in 1° increments of the angle formed by the ligand, the Fe center, and nitrogen of the axial His from 90° to 180° (Supporting Information Figure ). Reaction coordinates for hydrogen atom transfer (HAT) and radical rebound were obtained at the B3LYP-D3/6-31G* level of theory. The B3LYP/6-31G* method/basis set combination was chosen to reduce the computational cost associated with the larger def2-TZVP basis set due to increased system size from the presence of substrate atoms in these calculations and for consistency with prior work. Hydrogen atom transfer (HAT) RCs were obtained using Fe(IV)=O and lysine, which is the substrate of a representative non-heme iron halogenase, BesD . The radical rebound RCs (i.e., with both hydroxo and halogen F, Cl, or Br) were modeled following prior work as the rebound of a 2-methylbutane radical to an Fe(III)-OH intermediate, where 2-methylbutane was chosen as the model substrate because it closely resembles most substrates of Fe(II)/aKGdependent enzymes (Supporting Information Figure ).