Molecular adsorption on silicon (001): A systematic evaluation of size effects in slab and cluster models

First-principles calculations are in wide use today to describe chemical processes occurring on the silicon (001) surface. The number of atoms that can be explicitly treated is limited and hence size-constraints are invariably required;this applies to both cluster and periodic slab approaches. Using a trial set of seven molecular adsorbateconfigurations, we examine the dependence of calculated adsorption energies on several size parameters, namely thickness and in-plane unit cell size for slab models, as well as thickness, length, and width for cluster models. Size-converged adsorption energies are estimated by extrapolation, and are used to assess the accuracy of the more typically-sized slabs and clusters in common use today. Use of a DFT method that can be applied to both slabs and clusters allows us to assess the performance of these two approaches on an equal footing.