1. As examples of these seeS. Baer:Physica A (The Hague),87, 569(1977);91, 603 (1978).
2. S. Franchetti:Nuovo Cimento B,55, 335, 348 (1968);10, 211 (1972);42, 85 (1977), henceforth referred to, in the order, as I, II, III, IV.
3. A. Rahman:Phys. Rev. Sect. A,136, 405 (1964). 4
4. A recent review of this work has been given byA. V. Chadwick andH. E. Glyde: inRare Gas Solids, edited byM. L. Klein andJ. A. Venables, 2nd vol. (New York, N. Y., 1977), p. 1152.
5. This assumption affects the value of IIcross (see below) by affecting the acceptance angle, i.e. the maximum value of 6 for trajectories reaching π (for a given initial velocity). Inspection of about one-hundred computer-simulated trajectories has shown that they are often far from rectilinear. However, in the important region (corresponding to the maximum of the curve in fig. 10) the effect should not exceed about 10%. (In the said region 6 has a value of only 11.3°.)