The arrangement of the atomic building blocks on solid surfaces may considerably differ from what is to be expected from the crystalline (bulk) solid cut along the corresponding crystallographic plane. These deviations are classified into relaxations and reconstructions, respectively. In the lecture, there will be explained how such surfaces can be characterized uniquely in excess to the conventional Miller indices.

The basic reason for the occurence of such phenomena is minimization of the corresponding thermodynamic potential in thermodynamic equilibrium. Actually, the development of surface structures is often driven by the simultaneous reconstruction of the surface electronic states exhibiting some similarity to phenomena as Peierls transitions or Migdal-Kohn anomalies.

Adsorbate structures develop due to a refined energetic interplay between adsorbate-adsorbate, adsorbate-substrate and indirect (via the substrate) adsorbate-adsorbate interactions. Examples from our own work on ordered adsorbate layers of organic molecules on crystalline substrate will illustrate this topic.

Model for the commensurate adsorption geometry of F₁₆CoPc/Ag (110) [M.Toader, T.G.Gopakumar, M. Abdel-Hafiez, M.Hietschold. to be published]