IMPORTANCE OF MADELUNG POTENTIAL IN QUANTUM-CHEMICAL MODELING OF IONIC SURFACES

The importance of the inclusion of the Madelung potential in cluster m odels of ionic surfaces is the subject of this work. We have determine d Hartree-Fock all electron wave functions for a series of MgO cluster s with and without a large array of surrounding point charges (PC) cho sen to reproduce the Madelung potential. The phenomena investigated in clude: the reactivity of surface oxygens toward CO2, atomic hydrogen, and H+; the geometry and adsorption energy of water and the vibrationa l 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 ind ependent of the presence of the PCs, other clusters, typically of smal l size, give physically incorrect results when the PCs are not include d. The embedding of the cluster in PCs guarantees a similar response f or clusters of different size, at variance with the bare clusters, whe re the long range coulombic interactions are not included. (C) 1997 by John Wiley & Sons, Inc.