The nature of the chemical bond in simple oxides: A theoretical journey from the ionic model to the ab initio configuration interaction approach

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Abstract
Ab initio cluster model wave functions of increasing complexity have been obtained for alkaline‐earth oxides MgO to BaO. Using a wave function corresponding to the superposition of the electronic densities of the cations and anions obtained in a Madelung field, an ab initio version of the ionic model is obtained. This simple ionic model is improved with self‐consistent field (SCF) and large multireference configuration interaction (CI) wave functions. Analysis of these different types of wave functions shows that the ground state of these oxides is strongly ionic with the ideally ionic configuration having a weight of ≊95% in the total CI wave function. With all the criteria that we have used, the degree of charge transfer from O2− to M2+ is always very small. Furthermore, the instantaneous electron–electron interactions (correlation effects) treated in the CI wave function have been found to be mainly intra‐atomic and especially important for the 2p electrons of O2−. Point charges were used to represent the contribution to the Madelung field made by the atoms not explicitly included in the cluster; they were chosen to reproduce the Madelung field arising when a fully ionic crystal is assumed. Sets of scaled point charges which correspond to a smaller Madelung field were also used. The cluster model results were not significantly changed when the point charges were reduced by as much as a factor of 2 from the values for the fully ionic crystal. This is strong evidence that the ionicity of the crystals results from chemical forces and is not due to the use of an assumed Madelung field external to the cluster.