Articles you may be interested inA new model of chemical bonding in ionic melts J. Chem. Phys. 136, 164112 (2012); 10.1063/1.4705668Bound states of positron with urea and acetone molecules using configuration interaction ab initio molecular orbital approach Structures of the aluminum oxides studied by ab initio methods with natural bond orbital analysis 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 0 2 -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 0 2 -. 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.
The electronic structure of the ground electronic state and of some special charge-transfer excited states in ionic solids is examined from the ab initio cluster model approach. Different ab initio wave functions, including a frozen orbital approach, the Hartree-Fock self-consistent field, and multireference configuration interaction wave functions, are considered and analyzed using different theoretical techniques. We explicitly consider some alkaline-earth oxides such as CaO, a more difficult case such as Al2O3, a transitionmetal oxide such as NiO, and a system with a more complicated structure such as KNiF3. Analysis of ab initio wave functions in terms of valence bond components shows that all these compounds are largely ionic, thus supporting the simple picture arising from the ionic model. However, the nature of the excited states is more complex. Alkaline-earth oxides lowest excited states are essentially described as charge-transfer excitations dominated by a single resonant valence bond structure and the calculated energy difference is comparable to the experimental optical gap. In the case of Al2O3, the electronic spectra presents excitonic features and the local charge-transfer excitation excited states provide a reasonable representation of these phenomena. Finally, several different valence bond structures are present in the lowest electronic states of KNiF3.
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