TOPOLOGICAL ANALYSIS OF CHARGE-DENSITY IN IONIC SOLIDS

Topological analysis of charge density is presented for cluster models simulating ionic oxides such as MgO and Al2O3. Use Of the constrained space orbital variation method for the clusters embedded in different Madelung fields permits us to develop the energy in various physical meaningful contributions and establish that our results are not biased to the ionic description. Although the energy decrease due to charge donation from the anions to the cation is somehow larger for Al2O3 tha n for MgO, both Laplacian maps and integration of charge density on th e topological atomic regions are consistent with an ionic picture of t he chemical bond in both compounds. Moreover, integrated net charges a re very close to the full ionic value. In summary, topological analysi s strongly suggests that these two oxides have to be regarded as essen tially ionic solids with very small participation of covalent effects.