Reassessing DFT Rankings for Ambimodal Cycloadditions: The Impact of Accurate CCSD(T) Benchmarks and New Doubly-Hybrid Functionals

The accuracy of reference data is pivotal, especially when they are used to benchmark modern density functional approximations (DFAs) that approach chemical accuracy. This work re-examines a recent benchmark study on an ambimodal cycloaddition reaction. Using canonical coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] as a rigorous reference, we demonstrate that the performance ranking of leading DFAs is highly sensitive to the quality of the reference energies. Among the tested DFAs, the XYG3-type doubly hybrid (xDH) functional XYG7 emerges as the top performer against this robust benchmark. For strongly correlated subsets (GB1 > 12), CASPT2 is employed as a complementary reference to canonical CCSD(T). The renormalized doubly hybrid functional R-xDH7-SCC15 shows consistent performance across both reference frameworks. This cross-validation highlights the limitations of relying exclusively on single-reference benchmarks and underscores the urgent need for high-accuracy multireference reference data to properly evaluate next-generation density functionals.