PERIODIC HARTREE-FOCK STUDIES OF THE PROPERTIES OF THE CL2/MGO (001) INTERFACE

The properties of the Cl2/MgO (001) interface were investigated using the ab initio periodic Hartree-Fock LCAO method. The calculations treated a system that was periodic in two dimensions with the adsorbate aligned perpendicular to a three-layer slab of MgO (001). The total energy was calculated as the periodic net of Cl2 molecules approached either an oxygen or magnesium site on the surface. The finite thickness of the slab was set such that the interlayer atoms resembled those found in the bulk. The basis set for the solid has an optimized split valence form with 8-61G on Mg and 8-51G on O. The binding energy is reported as a function of Cl2 surface coverage, with a 6-21G* basis set on Cl, in the range from 1:1 to 1:8. At the most dilute coverage (1:8), the Cl2 molecule is bound over an oxygen with a binding energy of 4.1 kcal/mol. The energetics of Cl2 approaching a magnesium show a purely repulsive interaction at all coverages. The interfacial properties are probed with calculations of the band structure, total and projected density of states, density deformation maps, Mulliken analyses, and electrostatic potential maps. The charge density and density deformation maps indicate that the binding of the adsorbate involves very little charge transfer (only approximately 0.03 \e\) between the surface and the molecule. The calculated electrostatic potential and the electric field maps reveal that the interaction is dominated by the electrostatic interaction between the molecule and the surface. A simple electrostatic description of the attractive forces in the binding energy can account for the preference of the adsorbate to bind over oxygen. The total and projected density of states indicate that the higher energy valence states in the slab are dominated by contributions from the 2s and 2p orbitals centered on the oxygens. These states overlap and mix with the states resulting from the 3s and 3p orbitals of the Cl2 net. This mixing occurs to a much smaller extent between chlorine and magnesium orbitals.