ASSOCIATED CONTENT Supporting Information. Extracted active site of WelO5 with three halides, i.e., F, Cl, Br; active site isomers and constrained atoms, angles, and distances; differences between implicit solvent and gas-phase metal-ligand bond lengths; potential spin states of active site intermediates with three halides (F, Cl, Br); tight PNO thresholds used in DLPNO-CCSD(T)/CBS calculations; active site isomers of intermediates formed during catalytic cycle; DLPNO-CCSD(T)/CBS spin-splitting energies of oxo and OH intermediates; differences between high-spin and low-spin metal-ligand bond lengths; relative energies of isomers obtained with DLPNO-CCSD(T)/CBS; optimized geometries of Fe(IV)=O isomers with three halides (F, Cl, Br); optimized geometries of Fe(III)-OH isomers with three halides (F, Cl, Br); optimized metal-ligand bond lengths of all active site isomers; covalent radii of fluorine, chlorine, iron, and bromine atoms; Fe-Cl and Fe-Br bond lengths in optimized geometries and crystal structures; optimized geometries of Fe(II)-H2O and Fe(III)-O2 isomers; NHis-Fe=O angles in transition state geometries along isomerization RC; RC for isomerization between axial and equatorial hydroxo; relaxed RCs and RCs obtained as singlepoints along relaxed RCs; O•••H distances in high-energy structures along HAT RCs; O•••H distances in optimized transition state geometries along HAT RCs; optimized geometries of transition states along HAT RCs; relative energetics of halide-rebound and hydroxo-rebound intermediates (PDF)