COMPARISON OF THE BINDING OF CARBON, NITROGEN, AND OXYGEN-ATOMS TO SINGLE NICKEL ATOMS AND TO NICKEL SURFACES

Multireference CCI calculations have been performed for the diatomic molecules NiC, NiN, and NiO, and for the cluster systems NiSC, NiSN, and Ni50. Of the diatomic molecules, NiN has by far the smallest dissociation energy, which we argue is primarily due to the small electron affinity of nitrogen. The bonding to nickel 3d, which is pronounced for the diatomic molecules, disappears almost entirely for the cluster systems. In the full geometry optimization of the NiSX systems, including cluster relaxation, the final geometries are rather similar for the three adsorbates, all being adsorbed below the surface. The energy gain in penetrating the surface is much larger for carbon and nitrogen than it is for oxygen, however. This is in line with the experimentally determined surface geometries, where carbon and nitrogen adsorb close to the surface and reconstruct the surface, whereas oxygen stays further out and does not reconstruct the surface. For oxygen the restoring forces from neighboring atoms in the surface are larger than the driving force to reconstruct the surface. The origin of the difference between the adsorbates is that the 2p, orbital, which points toward the surface, is almost doubly occupied for oxygen, whereas this orbital is only partially occupied for carbon and nitrogen