Surface-localized ionic clusters

All theoretical studies agree with this second picture [13-17,39,99]. This is shown by the plots of the electron density maps and in particular by density difference maps which show very clearly the electron localization at the center of the vacancy [39,55]. This is true not only for the bulk but also for the surface of MgO, Fig. 5. The localization of the electrons in the center of the vacancy is an indirect proof of the highly ionic nature of MgO. In fact, the electrons are trapped in the cavity by the crystalline Madelung potential. Calculations performed on cluster models have shown that in absence of the external field the electrons tend to distribute more over the 3s levels of the Mg ions around the vacancy [38]. The localization of the electron in the center of the vacancy is... 

An a priori analysis on the reactivity and peculiarities of chemical behavior of molecules is a rather difficult but quite solvable task of theoretical chemistry. If molecules interact with a sohd surface, the complexity of its solution increases repeatedly. This is cause by the circumstances as follows firstly, an interaction occurs between two systems of different nature — molecule and surface that can be considered to be endless at the scale of partner secondly, it is difficult to simulate a surface adequately that is a macrodefect of the crystal periodic structure. Moreover, a definite grade of amorphization of surface layer is a characteristic of even typical crystal [125]. Taking into account probable relaxation and reconstruction of real surface as compared with ideal one, obtaining valid structural information on surface and subsurface layer of solids seems to be rather problematic. A cluster model of solid and its surface that is natural for chemists operating terms of local chemical bonds (despite that it is not quite suitable for the systems with covalent bond) may be considered to be fit for the objects with ionic bonds that are objects of our investigation. 

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