where, E' represents the absolute energies, x 0 represents the minimum energy point. And, the RMSE and MUE were evaluated with the relative energies. For calculating the RMSE of thermally relevant low-energy region (TRLR), only the relative energies below 5 kcal/mol on the reference energy surface were chosen. The cutoff of 5 kcal/mol was chosen to favor low-energy regions in state space. Table : Overall RMSE and MUE (in kcal/mol) in torsion profile energies of the molecule set with respect to the reference level CCSD(T)/[haTQZ; δ:haDZ]//MP2/heavy-aug-cc-pVTZ level of theory. The 95% confidence intervals, calculated with cinnabar, were presented on the side. Furthermore, the last column includes the RMSE within the thermally relevant low-energy region (TRLR) of energies < 5 kcal/mol, averaged over all the molecules, which serves as a metric for assessing accuracy in low-energy regions. The best performer on this set of molecules is ωB97M-D3(BJ)/DZVP, and our current choice of theory level, B3LYP-D3(BJ)/DZVP, lags behind it by only 0. The RMSEs on the whole set and the subsets of neutral and charged molecule sets were depicted in Figure and tabulated in Table . We can see from Figure (f), and from Table , that the accuracy of B3LYP-D3(BJ) functional increases for charged molecules with addition of polarization and diffuse functions from DZVP to def2-TZVP and higher.