Fig. 21. Potential energy surface for the translation of a rigid PTCDA molecule 2.9 Å above the Ag(111) surface, calculated for a 6 × 6 supercell using PBE+vdWsurf, as obtained by Hörmann et al.282 v1 and v2 denote the primitive lattice vectors of the substrate, z the adsorption height and φ the rotation angle of the molecule relative to the substrate. The potential energy surface shows a minimum on the Ag-bridge site, a low-energy saddle point along v1 and a high-energy saddle point along v2.
