**QC methods and band structure** Hartree Fock calculations tends to localise electrons and give rise to insulators. The metal Al for example is an insulator in HF. Is this due to the fact that HF can only use a single determinant and that not multiple determinants with fractional occupation numbers can be used to simulate the system.

Pure DFT tends to give rise to metals. For instance, the semi-conductor NiO is a metal in DFT. This is due to the fact that DFT suffers from self interaction. The term <ii||ii> does not vanish as in HF theory and therefore the electron 'sees itself'. It leads to a higher electron repulsion and hence a higher delocalised behavior.

Hybrid DFT functionals tend to give the correct behavior as correlation effects change the wavefunction sufficiently to give the correct description of the electrons. It gives rise to correct band structures. Mostly.

__See also:__ HF band gap

http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1198498430/0#0