We distinguish three categories of adsorption geometries based on which molecular axis is perpendicular to the surface (see Figure ). There are four geometries with the x-axis perpendicular to the surface ("standing-x"), four with the y-axis perpendicular to the surface ("standing-y") and three with the z-axis perpendicular to the surface ("lying"). Standing-x and standing-y category geometries differ in the orientation of the C --C backbone of the adsorbate relative to the surface. For x, it is perpendicular to the surface; for y, it is parallel. For the geometries of the lying category, all four nitrogen atoms are in contact with the surface. For the two standing categories only two nitrogen atoms bind to the surface, with the π-plane of the molecule being perpendicular to the surface.