A modified-local-harmonic model for solids

We present a modified-local-harmonic model for solids that permits quick and accurate calculations of the finite-temperature properties of perfect crystals and defects. The results obtained with this simple modification are more accurate than those obtained by using the local harmonic approximation, and this approach still retains the simplicity inherent in the local-harmonic approximation. The utility of this new approach is demonstrated by a calculation of the free energy of a perfect solid and the formation free energy of an isolated point defect and a comparison of the results with those obtained by using the local-harmonic and quasiharmonic approximations. Finally, we relate both the local-harmonic and the modified-local-harmonic models to a model based on a local vibrational density of states that has been proposed by Sutton.