(3.5.1) Perdew : Given the exact electron density for a real system, and excluding exotic cases like the strongly stretched hydrogen molecule ion, the meta-GGA level of nonlocality can often give an accurate energy. But the level of nonlocality of the exchange-correlation potential (functional derivative of the exchange-correlation energy) can be much more critical for the electron density. The meta-GGA density is not sufficiently localized around the nuclear centres for some chemical problems, where the Hartree-Fock density, which comes from a more nonlocal exchange-correlation potential, is better. In a solid metal, however, the Hartree-Fock and even the meta-GGA density may be too localized in comparison with the exact density. Importantly, an approximate functional that is accurate for the energies on the exact electron densities of a wide class of real systems can still have inaccurate functional derivatives and thus inaccurate self-consistent densities in that class.