DLPNO-CCSD(T) Calculations. DLPNO-CCSD(T) calculations were performed on TPSShoptimized geometries, using TPSSh reference orbitals. The iterative triples correction of Guo et al. was employed, as it yields better relative energies for transition metal systems and other molecules with low-lying excited states. Due to the large disk-space demands of this procedure, the cutoff for including triple natural orbitals (TCutTNO) was increased from 1.0 × 10 -9 to 5.0 × 10 -9 . This allowed all calculations to be performed on 2 TB of local disk space. Benchmarking illustrated that this change had minimal effect on the accuracy of the method (Section S2), which is consistent with earlier benchmarking done by Guo et al. For calculations of ∆E O and ∆E OH , and ∆E OOH two-point complete basis set (CBS) extrap-olation was performed using the aug-cc-pvdz and aug-cc-pvtz basis sets on all atoms using the procedure and fitting parameters of Neese and Valeev. In addition, an extrapolation to the complete pair natural orbital space (CPS) was performed using the CPS1 variant procedure described by Drosou et al., which was shown to greatly improve spin-splitting energies in iron complexes. In all cases, the NormalPNO setting was employed, though the TCutPNO setting was modified for the CPS extrapolation. For calculating the lowest energy spin states, the same CBS/CPS procedure was used. However, only the basis set of the metal atom was extrapolated, which improved efficiency while accurately treating the orbitals important for determining multiplicity. These calculations used the following basis sets: def2-SVP for H and C, def2-TZVP for N and O, and def2-TZVPP / def2-QZVPP for the metal.