As explained below, we find that standing-x interacts with the surface so weakly, that it plays no role in the observable polymorphism. This agrees with earlier findings (see Egger et al., where standing-x adsorption geometries were neglected in the structure search). We also find that standing-x and standing-y are affected by the field in a very similar way. Therefore, we only show differences between lying and standing-y hereafter. For the sake of brevity, we will also shorten the name of category "standing-y" to "standing". We show the most stable adsorption geometries for the categories lying and standing in Figure .