  We consider an isolated point defect embedded in a homogeneous crystalline
solid. We show that, in the harmonic approximation, a periodic supercell
approximation of the formation free energy as well as of the transition rate
between two stable configurations converge as the cell size tends to infinity.
We characterise the limits and establish sharp convergence rates. Both cases
can be reduced to a careful renormalisation analysis of the vibrational entropy
difference, which is achieved by identifying an underlying spatial
decomposition.
