1. A.U. MacRae and G.W. Gobeli, in Semiconductors and Semimetals, Vol. 2, Eds. R.K. Williams and A.C. Beer ( Academic Press, New York, 1966 ) p. 115.
2. R.J. Meyer, C.B. Duke, A. Paton, A. Kahn, E. So, J.L. Yeh, and P. Mark, Phys. Rev. B19. 5194 (1979) and references given therein.
3. D.J. Chadi, Phys. Rev. Lett. 41, 1062 (1978).
4. D. Haneman, Phys. Rev. 121, 1093 (1961) and 170, 705 (1968).
5. D.E. Eastman, T.-C. Chiang, P. Heimann, and F.J. Himpsel, Phys. Rev. Lett. 45, 656 (1980).