Unlike for the UW12 functional, upon re-optimization, the weighted RMSE of the training set increases compared to the initial orbital version going from 5.49 kcal mol -1 to 6.45 kcal mol -1 in the final functional. A complete summary of the RMSEs for each dataset in the training data at each stage in the optimization of ωB97X21-D3(BJ) is shown in the supporting information. Of the 24 datasets only 9 have reduced errors on re-optimization, though 6 of these errors are reduced by more than 0.1 kcal mol -1 . Large increases of more than 0.1 kcal mol -1 in the RMSE are observed in 10 of the sets, with the greatest increases being for the AE18 atomization energies set, the CRBH20 barrier heights set and the G21EA electron affinity sets which all increase by more than 0.5 kcal mol -1 . The RMSE for the atomization energy set AE18 increases by 2.19 kcal mol -1 , by far the greatest increase. The cause of this increase is most likely due to the small amount of same-spin correlation in the resulting functional, Table : Mean absolute errors (MAEs) in kcal mol -1 for each of the datasets in the testing set of MGCDB84. The minimum and maximum errors for each set are highlighted in green and red respectively. with the amount of same-spin correlation greatly reduced in the orbitals compared to the initial ωB97X orbitals.