Effect of the surface model on the theoretical description of the chemisorption of atomic hydrogen on Cu(001)

Adsorption at surfaces can be modelled using a periodic supercell approach or using finite clusters. For many systems and properties these models are complementary and often the most productive way to work is to use a combination of these techniques. If reliable data is to be obtained it is essential that convergence is achieved with respect to the size of supercell and cluster. This work discusses the convergence of chemisorption properties of H on Cu(001) with respect to the cluster size. To this end calculations of the H binding energy and equilibrium distance, are reported for cluster models of increasing size containing up to 77 metal atoms. Likewise, periodic slab model calculations are used to provide the corresponding values towards which the cluster approach should converge. In many previous studies of a wide variety of systems it has been established that computed equilibrium distances converge rapidly with respect to cluster size. Here, a systematic study of the dependence on cluster size shows that, for adsorption in the 4-fold site, convergence is not achieved even for very large clusters. The reason for this poor convergence is seen to be the inability of the cluster model to reproduce accurately the charge density and electrostatic potential of the crystalline surface.