NH3 on Si(0 0 1): Can Gaussian cluster and planewave slab models agree on energetics?

We critically evaluate the use of cluster and periodic slab models in describing the NH3/Si(0 0 1) molecule-surface reaction system. We show that considerable discrepancies in the relative adsorbate energetics originate in the limitations of the small cluster and slab models commonly used. These limitations in turn are the consequence of the balance that must be struck between the competing demands of cluster/slab size, basis set size and exchange-correlation model. This leads us to consider “cluster compound models” in which the results of several smaller calculations (separately probing the effects of cluster/slab size, basis set size and exchange-correlation model) are combined to estimate the energy of a converged model.