(3.4.4) Ayers : Multireference effects are obviously problematic in single-determinant theories like Kohn-Sham DFT. But even if one changed to a different starting point (perhaps by moving to an extended DFT), there would still be (different) types of correlation that would be difficult to describe. Indeed, for every practical electronic-structure method I know, I can think of some type of electron correlation that it struggles to describe, so ab initio DFT 356 is no solution. I would not say the situation is hopeless, but I accept that different flavours of DFAs will be needed for different types of properties and systems. Focusing on my own experiences, the widely used B3LYP functional is among the best DFAs for studies of the activation barriers that determine aerobic oxidations of alcohols by gold nanoparticles, 298 the BP86 and B97-D functionals performing considerably worse. At the same time, B3LYP and other hybrid functionals work poorly and the BP86 (especially when corrected for dispersion) and B97-D functionals are impressively accurate in applications of DFT to methyl-cobalt bond dissociation and low-lying excited states of methylcobalamin. The latter situation is similar to that created by the application of various DFAs to dicopper-dioxygen structural motifs. For example, when examining isomerization curves connecting the bis(µ-oxo) and µ-η 2 :η 2 -peroxo isomers of Cu 2 O 2 cores supported by 0, 2, 4, and 6 ammonia ligands, hybrid functionals fail, the magnitude of the error being directly proportional to the percentage Hartree-Fock exchange in the functional. Pure GGAs work well in this case.