If we replace the error criteria for the binding energies in weakly bound species and activation energies by relative errors of, say, 5%, then we may be in a better position to judge and make recommendations regarding what method to use. Indeed, if the activation energies along the reaction pathway of interest are on the order of 10 kcal/mol or more, as in ref. 298, then ∼5% relative errors translate into total errors on the order of 0.5-1 kcal/mol, which is good enough to understand the reaction mechanism. However, if the activation barriers are on the order of 1-5 kcal/mol, or if we must decide if a particular reaction has a barrier or is barrierless, as in ref. 299, then a fixed accuracy criterion may be insufficient to make a proper recommendation regarding what method to use. Similarly, a fixed 0.1 kcal/mol criterion might be of little use for some noncovalent interactions, where there are many cases in which the interaction energies are as small as 1 kcal/mol or less and equally many cases in which the interaction energies are on the order of a few kcal/mol or more. A good illustration of the former situation is the magnesium dimer, which is an important weakly bound system in studies of ultracold and collisional phenomena and which is characterized by a binding energy of about 430 cm -1 ; see refs. 300,301. To properly understand this system, one must be able to reach an accuracy on the order of a few cm -1 , which is a major undertaking, even for the highest levels of ab initio wave-function theory. The magnesium-dimer example is also a reminder that in setting accuracy targets, we should be careful about treating wavefunction methods, including those based on coupled-cluster theory, as providers of reliable reference information. As is very well known, wave-function methods exhibit a much slower convergence with the basis set than methods based on DFT. Also, the treatment of core electrons is usually different in DFT and wavefunction calculations. Wave-function calculations are often carried out with frozen core electrons, whereas DFT (putting aside the issue of relativistic effective core potentials) is an all-electron theory. Thus, judging DFAs by comparison with wave-function approaches may sometimes be misleading or questionable. Finally, the CCSD(T) approach, often regarded as a standard for high-accuracy calculations, fails not only in multireference situations, such as covalent bond stretching and biradicals, but also in many cases of noncovalent interactions, including the aforementioned magnesium dimer, where the CCSD(T) binding energy extrapolated to the complete basis-set limit has a substantial error. While the development of ab initio wave function methods can be well served by comparisons with full CI, the development of DFAs may be better served by comparisons with reliable experimental data.