Predicted modifications in the direct and indirect gaps of Ge

The charge density of Ge was studied at various k-points and for various bands, by the ab initio pseudopotential method, using additionally the interstitial sites. The lowest X_c conduction-band points were found to be unique in having a high charge density in the interstitial site. It has been therefore predicted and verified that the X_c points move up in energy relative to the Γ_c point when closed-shell atoms (like H) are substituted at the interstitial sites. The calculations also indicate the change of the band-gap for HGeH.