It is known from experiments that under ultra  high vacuum (UHV) conditions, at room temperature, dissociative adsorption of O₂ results in an (apparent) saturation coverage of half a monolayer of oxygen atoms on the surface. On the basis of DFT calculations we predicted that even higher coverages should be attainable and that their formation under UHV conditions is only kinetically hindered, namely, we predicted formation of a full monolayer structure. This phase was subsequently verified experimentally. In order to achieve this, use of an oxygen carrying molecule that readily dissociates in the presence of O on the surface is necessary e.g. NO₂, or high gas pressures of O₂ needs to be employed.  Identification of this phase illustrates the different behavior of a system under UHV and high pressure conditions (the pressure-gap) and  also immediately raises the question of the possible existence of other high coverage surface structures
likewise achievable by bypassing the ``pressure gap' which may be of importance in the understanding of heterogeneous catalysis.[C.Stampfl, S. Schwegmann, H. Over, M. Scheffler, and G. Ertl, Phys. Rev. Lett. 77 3371 (1996)]