We use an embedded-cluster approach to study the lattice relaxation induced by a substitutional ${\mathrm{Cu}}^{+}$ impurity in alkali halides. The crystalline lattice is modeled as a quantum-mechanical molecular cluster embedded in a classical lattice. The cluster is treated by using the unrestricted Hartree-Fock approximation and the embedding lattice is described by the shell model. The relaxations are obtained by comparing the equilibrium configurations of the pure and ${\mathrm{Cu}}^{+}$-doped clusters. We found that, when the ${\mathrm{Cu}}^{+}$ ion is introduced into the alkali halides, the lattice relaxes in accordance with the free-ion size difference between the ${\mathrm{Cu}}^{+}$ ion and the host cation. The correlation correction does not make a significant contribution to the lattice relaxation.

• Received 16 September 1991

DOI:https://doi.org/10.1103/PhysRevB.45.2709