1. Useful references may be found in the following reviews: (a) Harrison, W.A.: Pseudopotentials in the Theory of Metals, New York: Benjamin 1966; (b) Heine, V., Weaire, D.: Solid State Physics, ed. Seitz, F., Turnbull, D., and Ehrenreich, H., New York: Academic Press 1970, vol. 24, p. 242; Cohen, M.H., Heine, V.: ibid p. 50; (c) Joshi, S.K., Rajagopal, A.K.: Solid State Physics, ed. Seitz, F., Turnbull, D., and Ehrenreich, H., New York: Academic Press 1969, vol. 22, p. 159
2. Hafner, J., Schmuck, P.: Phys. Rev. B9, 4138 (1974)
3. Hafner, J.: Z. Physik B22, 351 (1975)
4. Hafner, J., Eschrig, H.: phys. stat. solidi (b)72, 179 (1975)
5. Hafner, J.: J. Phys. F5, 1439 (1975)