A technique for embedding clusters of ions is presented,which are treated by the GAUSSIAN 94(G94) Quantum Chemical Program,in ionic crystals.The electrons and the nuclei of the clusters are in the Coulomb field created by the external infinite lattice.The Coulomb potential is calculated by the method of Ewald.The one-electron Hamilton array and the total energy of the cluster are revised.The method is illustrated by considering the NaF:Cu⁺ system and the formation of the F center defect in NaF.The calculated vertical excitation energies for NaF:Cu⁺ and the excitation energy of the F center are in fair agreement with the experiments,showing that the technique is credible.