Importance of Madelung potential in quantum chemical modeling of ionic surfaces

The importance of the inclusion of the Madelung potential in cluster models of ionic surfaces is the subject of this work. We have determined Hartree-Fock all electron wave functions for a series of MgO clusters with and without a large array of surrounding point charges (PC) chosen to reproduce the Madelung potential. The phenomena investigated include: the reactivity of surface oxygens toward CO₂, atomic hydrogen, and H⁺; the geometry and adsorption energy of water and the vibrational shift of CO adsorbed at Mg²⁺ sites; the electronic structure and the hyperfine coupling constants of oxygen vacancies, the paramagnetic F_s⁺ centers. While some clusters give results which are virtually independent of the presence of the PCs, other clusters, typically of small size, give physically incorrect results when the PCs are not included. The embedding of the cluster in PCs guarantees a similar response for clusters of different size, at variance with the bare clusters, where the long range coulombic interactions are not included. © 1997 by John Wiley & Sons, Inc.