Among the tested functionals, the PBE0, M06-2X and MN15 are most accurate both for J and ΔEspin, with MAE of 11.74 meV, 12.66 meV and 10.64 meV for J and 0.34 eV, 0.32 eV and 0.31 eV for ΔEspin. The MRE are 15%, -30%, 0% for J and -4%, 29% and 21% for ΔEspin (Table and Figure ). The root mean square error (RMSE) and mean absolute relative error (MARE) are also shown in Table . Although the MAE of B3LYP and HSE06, i.e. 13.38 meV and 13.18 meV for J and 0.46 eV and 0.41 eV for ΔEspin, are comparable to that of PBE0 and M06-2X, they incorrectly predict the relative energy of closed-shell and high-spin state for TAM dimer, resulting in erroneous ΔEspin (Table ). These results demonstrate that PBE0, M06-2X and MN15