Supporting Information. List of refcodes retained and eliminated during data set curation; geometry criteria for fidelity evaluation of optimized geometries; list of SCO complexes for which gas-phase geometries failed fidelity evaluation; X-ray crystal structures of gas-phase optimized SCO complexes which failed fidelity evaluation; RMSD of optimized SCO complexes which failed fidelity evaluation; summary of 23 density functional approximations; comparison of LACVP* and def2-TZVP spin splitting energies; ligand types, coordinating atoms, and denticities of SCO complexes; representative SCO complexes with N, S, and O-ligand coordinating atoms; average metal-ligand bond lengths across different ligand denticities; average metal-ligand bond lengths of DFT geometries and crystal structures; average metal-ligand bond length differences; histograms of average metal-ligand bond length differences; histograms of average scaled metalligand bond length differences; histograms of average metal-ligand bond length differences; histograms of DEH-L at aHF = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30; histograms of DGH-L at aHF = 0.05, 0.15, 0.25; average and standard deviation of modB3LYP DEH-L and modB3LYP DGH-L; Xray structures of SCO complexes that are outliers on DGH-L histograms; histograms of DGH-L corrections, ZPE, thermal vibrational energy, and vibrational entropy at aHF = 0.00, 0.10, 0.20, 0.30; average ZPE, thermal vibrational energy, vibrational and rotational entropy; outliers observed in free energy corrections at different aHF values; histograms of DGH-L of GGA, meta-GGA, GGA hybrid, meta-GGA hybrid, range-separated hybrid, ans double hybrid DFAs; average and standard deviation of DEH-L and DGH-L obtained with 23 DFAs; list of complexes with GGA and meta-GGA predicted SCO behavior; X-ray crystal structures of GGA and meta-GGA predicted SCO complexes; histograms of DEH-L obtained with M06-L and TPSSh; SCO complexes not predicted by modB3LYP (aHF = 0.10), M06-L, TPSSh; X-ray crystal structures of complexes with conflicting SCO predictions; percentile ranks of DGH-L to quantify changes in SCO predictions; plots of percentile ranks of DGH-L and T1/2 values; list of SCO complexes with significantly different DGH-L percentile ranks; plots of experimentally reported and estimated experimental T1/2 values; complexes that exhibit two-step SCO behavior or lack experimental T1/2; mean and standard deviation of experimental low-T, high-T, T1/2; histograms of low-T and high-T of crystallization; SCO complexes with unusual low-T and high-T of crystallization; SCO complexes with identical reported and estimated T1/2; T1/2 predictions by modB3LYP fuunctionals and 23 DFAs; plots of experimental T1/2 vs T1/2 predicted by modB3LYP, M06-L, TPSSh; plots of estimated T1/2 vs T1/2 predicted by modB3LYP, M06-L, TPSSh; list of complexes with comparable experimental and DFA-predicted T1/2; SCO complexes with comparable experimental and DFA-predicted T1/2; list of complexes and X-ray structures of complexes where experimental and DFA-predicted T1/2 differ considerably; plots of experimental T1/2 and average DFA-predicted