Ab Initio Calculation of the Dispersion Interaction between a Polyaromatic Molecule and a Noble Metal Substrate: PTCDA on Ag(110)

The geometry of PTCDA adsorbed on a (110)-oriented silver crystal is optimized on a finite metal cluster used as a rigid substrate. In a comparison between second-order Moller-Plesset perturbation theory (MP2) and density functional theory (DFT), we find pronounced differences; MP2 gives a nearly flat adsorbate, in agreement with the geometries deduced from X-ray standing wave studies on similar substrates, whereas the lack of dispersion interactions in DFT results in a strongly bent geometry. In MP2, the van-der-Waals attraction counterbalances the overlap repulsion, resulting in an average height of the PTCDA adsorbate above the topmost substrate layer of 2.68 angstrom for C atoms, comparing favorably with recent measurements based on X-ray standing wave techniques, The carboxylic oxygens interact more strongly with the substrate than the anhydride groups, so that we predict a height difference of 0.13 angstrom between both types of oxygen atoms.