Electronic structure of negative oxygen ion in a MgO crystal

An oxygen ion in a perfect MgO crystal is considered. First, calculations o f the electronic structure of the crystal in the Hartree-Fock approximation were performed. For this purpose, the noncanonical orbitals localized on t he ions were employed, which are solutions to the Hartree-Fock equations fo r ions in a confining potential simulating the crystal environment. The con fining potential was not set a priori but was developed in the process of c alculations designed to minimize the energy of the crystal. The cluster exp ansion for the crystal energy was employed. No additional approximation was used in calculating the n-ion contribution to the energy; the summation ov er the crystal was performed to the point of convergence (at a given precis ion). This is a new feature and an advantage of the present approach. A lar ge Cartesian Gaussian basis was used for each ion to ensure the calculation s were of the Hartree-Fock limit quality. It was found crucial for the conv ergence of the n-ion interaction energy to separate it into long- and short -range components and to perform the lattice summation for them independent ly. To take account of the intraionic correlations, the confining potential obtained at the Hartree-Fock stage was fixed and the configuration interac tion method was employed, the Sturm functions and the numerical technique b eing used. The calculated cohesive properties of MgO crystal are in good ag reement with experiment, the effect of intraionic correlations showing a co rrect tendency, though small.