1. Alibert, C., G. Bordure, A. Laugier, and J. Chevallier (1972), Phys. Rev. B 6, 1301.
2. Andersen, O.K., O. Jepson, and D. Glotzel (1985), in Highlights of Condensed Matter Theory, ed. F. Bassiani et al. ( North-Holland, Amsterdam).
3. Bachelet, G.B., D.R. Hamman, and M. Schluter (1982), Phys. Rev. B 26, 4199.
4. Bardeen, J., and W.H. Brattain (1948), Phys. Rev. 74, 230.
5. Brattain, W.H., and J. Bardeen (1948), Phys. Rev. 74, 231.