pi adsorption of ethene on to the {111} surface of copper - A periodic ab initio study of the effect of k-point sampling on the energy, atomic and electronic structure

The adsorption of ethene on the (111) surface of copper has been studied by using density functional theory calculations with gradient corrections. Th e surface is described by a periodic (3 x 3) slab, three layers thick, with ethene adsorbed on one side. The energy of the adsorption shows great sens itivity to the k-point sampling employed, with single k-point calculations overestimating the binding by over 800% when compared with a calculation co nverged with respect to the k-point sampling. In addition, the structure of the adsorbed molecule is considerably distorted, which is in contradiction with conclusions drawn from the experimental vibrational frequencies. Calculations that are converged with respect to the k-point sampling indica te a much weaker interaction between the molecule and the surface, with ads orption energies of 11.1 and 10.9 kJ mol(-1) for atop-h and atop-b, respect ively. This weaker interaction leads to a geometry for the adsorbed molecul e that is close to the gas-phase ethene structure, in agreement with the vi brational frequencies. We have proposed a model of molecular adsorption that is a balance between attraction, resulting from localised bond formation, and repulsion, due to interaction between the: extended electronic states and the molecule's elec tron density. If the extended electronic states are underestimated, as in c luster or low k-point calculations, the repulsion is underestimated. This r esults in stronger bonding to the surface and overestimation of the adsorpt ion energy. (C) 2000 Elsevier Science B.V. All rights reserved.