The low-energy and highly anharmonic quantum dynamics of isolated impurities in solids were analyzed by using path-integral Monte Carlo simulations. Interstitial O was studied as being a paradigm for such behavior. The question of the geometry of this defect was considered and, depending upon the potential (or impurity mass), there was found to be a classical regime (where the maximum probability density for the O nucleus was at the potential minimum) and another regime (associated with highly anharmonic potentials) where this was not so. The two regimes were separated by a sharp transition. The de-coupling of the many-nuclei problem into a 1-body Hamiltonian, in order to describe the low-energy dynamics, was also studied. It was found that the adiabatic potential which was obtained by relaxing all of the other degrees of freedom, at each value of the coordinate associated with low-energy motion, gave the best approximation to the full many-nuclei problem.

R.Ramírez, C.P.Herrero, E.Artacho, F.Ynduráin: Journal of Physics - Condensed Matter, 1997, 9[15], 3107-16