The active site of WelO5 (PDBID: 5IQS), a representative, carrier-protein-free nonheme iron halogenase, was extracted from the enzyme's crystal structure, following the protocol from prior work. The native halide, i.e., chloride, was replaced with fluoride and bromide to generate three active site models corresponding to the three halides (Supporting Information Figure ). Active site intermediates and corresponding isomers of these active site models containing water (i.e., as in the crystal structure), O2, oxo, hydroxo, and succinate (monodentate and bidentate) ligands were generated by modifying the crystal structure with molSimplify, which uses OpenBabel as a backend. Consistent with prior work, the metal-distal carboxylate oxygen of aKG and succinate ligands along with Nd atoms of His ligands were protonated using Avogadro v1.2.0 to ensure an overall neutral charge for the final active site models (Supporting Information Figure ). All hydrogen atoms added to the active site models were force-field optimized in Avogadro with the UFF force field while holding the heavy atoms fixed.