Next, we analyzed changes in calculated scaled metal-ligand bond lengths as a function of aHF in the modB3LYP functional 99 (Figure ). Upon increasing aHF, we observe an increase in predicted metal-ligand bond lengths for LS states and, to a lesser degree, HS states (Figure and Supporting Information Table and Figures ). Because B3LYP (aHF = 0.20) overestimated experimental low-T bond lengths, the aHF = 0.00 structures are in best agreement with experiment (Figure and Supporting Information Table and). In contrast, there is no optimal aHF for improving agreement between DFT of HS states and experimental high-T structures (Supporting Information Table and Figures ). The different effects on LS and HS bond lengths can be rationalized by the effect of HF exchange 152,153 on delocalization error. LS states are more delocalized than HS states and more sensitive to HF exchange, consistent with observations from prior studies . Thus, although cancellation of errors means that aHF = 0.00 can slightly improve upon standard B3LYP for the prediction of structural properties for LS states, we expect non-zero HF exchange admixture to be essential to describe the electronic structure.