THE EFFECT OF A CRYSTALLINE ENVIRONMENT ON CALCULATED ELECTRON DENSITIES

Central to the calculation of useful interatomic potentials is a reliable determination of the electron densities of interacting ions. When the ions are embedded in a crystalline environment, it is reasonable to expect the effect of the crystal to be important in such calculations. For certain ions (for example Mg2+ ions in MgO) the crystal has little effect on ion electron densities. Conversely, ions such as O2- can only exist as a result of the stabilizing influence of lattice potentials; consequently, they are significantly modified by different lattice environments. This paper will discuss the relevance of lattice effects to ions in MgO, in particular, M2+, Mg+, O2- and F-. Of these examples, the defect ions Mg+ and F- are used to demonstrate how, in certain circumstances, the usual assumption that cations are not affected by lattice potentials but that anions are greatly influenced is not applicable. This work also emphasises the role of embedding electron-electron interactions in addition to the Madelung potential.