It was recalled that, in molecular dynamics simulations of amorphous solids, 2 distinct types of jump process were observed. These were collective jumps in which a single atom moved only a fraction of the interatomic distance, and jumps in which single atoms moved over interatomic distances. The 2 different jumps could interact via their long-range displacement fields. In the case of the molecular dynamics of undercooled liquid Zr67Cu33, it was noted that 2 processes contributed to diffusion and resembled the 2 types of jump in the solid. In addition to a more flux-like motion, the Cu atoms in particular jumped over nearest-neighbour distances. The latter process predominated, at least for Cu, at temperatures which were sufficiently far above the glass transition. With decreasing temperature, the jumps over atomic distances became increasingly reversible and their contribution to long-range diffusion decreased.

Diffusion and Relaxation in Liquid and Amorphous Metals. C.Gaukel, M.Kluge, H.R.Schober: Philosophical Magazine B, 1999, 79[11-12], 1907-14