The supercell method for calculating responses in defective lattices

The supercell method for calculating dynamical properties of defective crystal lattices is discussed and tested against the more conventional Green's function method in calculations of the frequency of the gap mode in Cl-doped KI. Both calculations are based on the same set of model potentials and they form a stringent test of these potentials in a defect dynamics application. Adequate convergence is not achieved with a supercell containing 64 atoms. Some results for a simple one-dimensional analogue system are presented in order to indicate how large supercells must be for convergence.