When excited states are considered and excitation energies are calculated, a quantitative treatment of electron correlations becomes vital. The point is that the correlation-energy contribution to the ground-state energy may be small compared with the dominating contributions of the self-consistent field; however, when energy differences with respect to the ground state are calculated, the changes in the correlation energy may become equal to or even larger than those resulting from changes in the self-consistent field. For example, in a semiconductor (or insulator) like diamond, the energy gap for exciting an electron from the valence into the conduction band is reduced by a factor of 1/2 due to correlations (Chap. 9).
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