The initial stage A. of defect creation, until all excess kinetic energy has dissipated in the lattice and it is back to its initial temperature T 0, takes < 5 ps. This is the fundamental ("primary damage") threshold displacement energy, and also the one usually simulated by molecular dynamics computer simulations. After this (stage B), however, close Frenkel pairs may be recombined by thermal processes. Since low-energy recoils just above the threshold only produce close Frenkel pairs, recombination is quite likely.