  We present a comparative study of metal-organic interface properties obtained
from dispersion corrected density functional theory calculations based on two
different approaches: the periodic slab supercell technique and cluster models
with 18 to 290 Ag atoms. Fermi smearing and fixing of cluster borders are
required to make the cluster calculation feasible and realistic. The considered
adsorption structure and energy of a PTCDA molecule on the Ag(110) surface is
not well reproduced with clusters containing only two metallic layers. However,
clusters with four layers of silver atoms and sufficient lateral extension
reproduce the adsorbate structure within 0.02 \AA\ and adsorption energies
within 10\% of the slab result. A consideration of the computational effort
shows that the cluster approach is a competitive alternative to methods using
periodic boundary conditions and of particular interest for research at surface
defects and other systems that do not show periodic symmetry.
