For each complex, low-, intermediate-, and high-spin structures were optimized, and the ground state spin was determined independently for each functional from the lowest single-point electronic energy of the three tested spin states. The full list of DFT-predicted multiplicities is given in Table . In general, these did not agree between functionals. Thus, the true ground state multiplicity for each intermediate was assigned using reference values from experiment or high-level wavefunction theory when available, or the lowest-energy state from our own DLPNO-CCSD(T) calculations otherwise. Neither of these options were available for NiP(O 2 ), but all tested functionals predicted a triplet ground state, which we assumed to be accurate.