Pseudopotential approach to the band structure of alkali halides

A pseudopotential approach to the calculation of the conduction band structure of ionic crystals is proposed along the lines suggested by Heine and Abarenkov. An improved model potential originally suggested by Griuliano and Ruggeri is adopted and discussed in connection with other choices of the potential. Detailed calculations are performed in LiCl, NaCl and KC1, and the results are similar to those obtained with the KKR and the OPW methods, but are more free electronlike, as suggested by Phillips. The minimum of the conduction band is F1 in NaCl and KC1 andL1 in LiCl. The lowest conduction band is nondegenerate and overlaps very little with the higher conduction bands. In the case of KC1 a well-defined secondary minimum exists atX3, while in LiCl and NaClX3 is more likely a saddle point. The strong absorbtion peak above ionization in KC1 is attributed to a resonant exciton associated to the minimum atX3, rather than to a peak in the density of states. The electron-hole interaction should be taken into account to interpret the entire optical excitation spectrum.