1. For a review see: Gadzuk, J.W. in: Photoemission and the Electronic Properties of Surfaces (ed. by Feuerbacher, B., Fitton, B. and Willis, R.). New York: Wiley 1978
2. Lang, N.D., Williams, A.R.: Phys. Rev. B16, 2408 (1977)
3. Schönhammer, K., Gunnarsson, O.: Solid State Communications,23, 691 (1977)
4. Schönhammer, K., Gunnarsson, O.: Solid State Comm., in press
5. Gunnarsson, O., Schönhammer, K.: Solid State Comm., in press