APE is started from six initial structures, containing from one to six O adsorbate atoms per surface unit-cell that are placed at the preferred adsorption sites first at the upper edge of one of the two steps contained in the surface unit-cell and once these sites are full at the fourfold hollow terrace sites next to these edge sites, cf. Fig. . With this range of stoichiometries, we can thus explore processes that emerge with increasing step decoration.