Comment on "Green's-function calculation of the lattice response near the vacancy in Si"

A fallacious claim is made by Lannoo and Allan [Phys. Rev. B 25, 4089 (1982)] that a soft phonon mode, which might result from a vacancy in Si, could produce a large entropy of formation for the vacancy in Si (>15kB). The fallacy results from the use of a formula that is valid only in the harmonic approximation and cannot be applied to the case of a soft mode. The actual value of the entropy associated with a soft mode, if there were one, would not exceed 3.4kB. It could be negative. © 1986 The American Physical Society.