The a b i n i t i o model potential representation of the crystalline environment. Theoretical study of the local distortion on NaCl:Cu+

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Abstract
In this paper we formulate a model potential approach to take into account the crystalline environment within the Hartree–Fock–Roothaan formalism. The formulation is based on the assumption that the theory of separability of many‐electron systems may be applicable to the group of electrons within a reference cluster and the groups of electrons on a set of external lattice sites which, in turn, can be represented according to the ab initio model potential method. The characteristics of the model potentials permit to analyze the contributions to the cluster energies and wave functions of different environmental effects, such as point‐charge and charge‐density Coulomb interactions and quantum interactions (exchange and orthogonality). The formalism is applied to the SCF calculation on the ground state of the octahedaral CuCl5−6 cluster (all‐electron calculation) embedded in a NaCl lattice which is represented by 118 model‐potential ions and 604 point‐charge ions. The calculation reveals that (i) the quantum interactions between the CuCl5−6 cluster and the rest of the lattice play an important role in determining the Cu–Cl distance and (ii) a considerable local distortion around the Cu+ impurity is predicted in which the Cl− ions move towards the Cu+ impurity about 0.1 Å. These results are in qualitative agreement with recent EXAFS studies; however, the predicted distortion is smaller than the one suggested by the EXAFS measurements.