  Only recently, the activation of oxygen molecules on clean defect-free
transition metal oxide surfaces has been reported, for example on the
CaO-terminated surface of the Ruddelsden-Popper perovskite \crosurf. In this
work we show that oxygen molecules adsorb as an activated superoxo species on a
clean SrO-terminated surface of \srosurf. At all coverages, the electrons
activating the molecule originate from the subsurface RuO$_2$ layer. At low
coverages, the presence of a Ca dopant in the terminating SrO layer slightly
increases the adsorption energy. At high coverage, DFT predicts a flat
potential energy surface and a preferred adsorption of the \oom\ near surface
cations. Advanced many-electron calculations (RPA) predict adsorption energies
of $-0.99$\,eV and $-0.49$\,eV per \oom~molecule for low and high coverages,
respectively, and a preference for forming line-like structures in the latter
case.
