Ab initio perturbed ion cluster-in-the-lattice calculations of the impurity centers NaI:Tl(+) and CsI:Tl(+) are
presented. We study several active clusters of increasing complexity and show that the lattice relaxation around
the Tl1 impurity implies the concerted movement of several shells of neighbors. The results also reveal the
importance of considering a set of ions that can respond to the geometrical displacements of the inner shells by
adapting self-consistently their wave functions. Comparison with other calculations involving comparatively
small active clusters serves to assert the significance of our conclusions. Contact with experiment is made by
calculating absorption energies. These are in excellent agreement with the experimental data for the most
realistic active clusters considered.
