1. I. M. Torrens, in: Interatomic Potentials (Academic Press, New York 1972).
2. A. A. Maradudin, E. W. Montrolli, G. H. Weiss, and L. P. Ipatova, Solid State Physics, Suppl. 3, 2nd Ed (Academic Press, New York 1971); G. Venkataraman, Dynamics of perfect Crystals (MIT Press, Cambridge, Mass. 1975).
3. See, for example, T. Kwok, P. S. Ho, and S. Yip, Phys. Rev. B 29, 5354 (1984).
4. K. Kunc and R. M. Martin, 6th General Conference of the EPS, Prague (1984).
5. J. Ihm, A. Zunger, and M. L. Cohen, J. Phys. C: Solid State Phys. 12, 4409 (1979).