RMSD (kcal/mol), for the DBH24 set of barrier heights, of double-hybrid functionals forms as a function of the percentages of MP2-type correlation (abscissa) and HF-type exchange (ordinate). Black dots indicate actual data points. Red squares indicate optima for specific subsets. The black line indicates the optimum `channel' for atomization energies. Becke88 GGA exchange, LYP GGA correlation, and the aug-pc3 basis set were used throughout.