The MgO(110) surface and CO adsorption thereon: I. Clean (110) surface

The MgO (110) surface has been studied using for the first time the exact-exchange Hartree-Fock approach within a crystalline-orbital ICAO formalism. The results obtained for slabs of different thicknesses (up to four planes) are presented and discussed. Unlike the (100) surface considered earlier, the symmetrical relaxation (compression) of the surface plane is found to be considerable (6.7% of the distance between two bulk planes) but the rumpling remains small (about 1.5% or less). The almost fully ionic character of the chemical bond, which was found earlier for bulk MgO and its (100) surface, is also maintained at the (110) surface in spite of a certain redistribution of the electronic charge at the surface. The surface energy (5.1 J m^-2) exceeds that obtained for the (100) surface by a factor of 2.5. The nature of the surface states is discussed by analysing the projected density of states.