Density functional cluster description of ionic materials: improved boundary conditions for MgO clusters with the help of cation model potentials

Ionic materials can be conveniently described by quantum mechanical cluster models. In these models the electrostatic field of the crystal surrounding is often taken into account by embedding the cluster in an array of point charges (PCs). However, anions at the cluster boundary can be strongly polarized by neighboring positive PCs. These artifacts can be significantly reduced if a model potential description is employed for the nearest-neighbor cations of the surrounding. We have implemented a scheme of total ion model potentials (TIMP) in the density functional code LCGTO-DF. In the present work we compare a number of electronic properties of MgO(001) models, like core-level splittings, electron affinity, and electrostatic potential, computed with the help of TIMP embedding to results obtained for the same clusters surrounded by PCs only. The cluster models range from MgO₅^8- to Mg₂₅O₂₅. Adsorption of a CO molecule on TIMP embedded cluster models of a regular surface site and the three-coordinated corner Mg²⁺ site is also considered.