Abstract
Three systems are considered which have large local field effects in their excited spectra. The first system is the absorption of soft x-ray radiation by the insulating crystal LiF, for which the lowest excitonic transition from the core state to the conduction band is forbidden. Next the optical properties of LiF in the 1-2-Ry energy range are examined. The last system discussed is NiO where the excitation spectra in the 0-20-eV energy range are investigated. In all three systems it is pointed out that a proper description must go beyond simple Hartree–Fock band theory and should include short-range correlation effects as well as the usual long-range correlations.
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