1. D. LOVERIDGE and S. PARK,Phys. Chem. Glasses 12 (1971) 19.
2. R. E. BEHRINGER,J. Chem. Phys. 29 (1958) 537.
3. J. M. VAN VLECK,Phys. Rev. 74 (1948) 1168.
4. D. GUIMARÃES and S. K. MENDIRATTA, unpublished work.
5. R. K. MACCRONE, J. WILLIAMS, K. KIM and M. P. MALEY, in ?The Structure of Non-crystalline Materials? Proceedings of the Symposium, Cambridge, 20?23 September 1976, edited by P. W. Gaskel, p. 121.