The entropy of Si self-diffusion during the concerted exchange process was calculated. The Vineyard transition-rate approach, and an accurate first-principles quantum-mechanical energy surface, were used. The high-temperature value of the entropy was estimated to be 3.3k. This was 2 or 3 times lower than the experimentally determined value. This relatively high entropy of the concerted exchange mechanism was attributed to the existence of multiple equivalent exchange paths, and to a combination of a stiff mode at the equilibrium configuration with a soft mode at the saddle-point configurations.

K.C.Pandey, E.Kaxiras: Physical Review Letters, 1991, 66[7], 915-8