Surprisingly, the second-best performing family is GGA. Despite the well-known underestimation of ΔET-Q by GGA, the average deviation from the CCSD(T) value is smaller than the other functional families. Overall, several functionals performed well for predicting the ΔET-Q and IE of HfCO (Figures and). Importantly, the best-performing functionals for HfCO De, the GGA hybrid B3LYP and the range-separated hybrid ωB97X, also had small errors for the IE (1.7 and 5.1%, respectively). The same cannot be said for ΔET-Q, where these functionals had 24.4 and 42.3% errors, respectively. Thus, our overall recommendation remains for the use of B3LYP and ωB97X based on De and IE despite some caveats regarding the spin state ordering. Nevertheless, it is noteworthy that all functionals correctly identify the ground state, despite in some cases underestimating (e.g., B3LYP) or overestimating (e.g., ωB97X) the quantitative value of the ΔET-Q gap (Figure ). values in molecules and provide a consistent result between the surface and the molecular complex.