The paper by Karsten Reuter and Matthias Scheffler investigates the composition, structure, and stability of the RuO$_2$(110) surface under different oxygen pressures using density-functional theory (DFT). The authors calculate the Gibbs free energy to determine the lowest-energy structure of the surface in thermodynamic equilibrium with an oxygen-rich environment. They find that the traditionally assumed stoichiometric termination is only favorable at low oxygen chemical potentials (low pressures and/or high temperatures). At realistic oxygen pressures, the surface is predicted to contain additional terminal oxygen atoms, forming a polar surface. However, the ionic model, which dismisses such terminations on electrostatic grounds, is shown to be invalid for RuO$_2$(110). The findings indicate that the stability of non-stoichiometric terminations is a more general phenomenon on transition metal oxide surfaces, similar to previous results for corundum-structured oxides. The study also discusses the importance of experimental preparation conditions, highlighting how the surface termination changes with oxygen pressure and its implications for catalytic reactions.The paper by Karsten Reuter and Matthias Scheffler investigates the composition, structure, and stability of the RuO$_2$(110) surface under different oxygen pressures using density-functional theory (DFT). The authors calculate the Gibbs free energy to determine the lowest-energy structure of the surface in thermodynamic equilibrium with an oxygen-rich environment. They find that the traditionally assumed stoichiometric termination is only favorable at low oxygen chemical potentials (low pressures and/or high temperatures). At realistic oxygen pressures, the surface is predicted to contain additional terminal oxygen atoms, forming a polar surface. However, the ionic model, which dismisses such terminations on electrostatic grounds, is shown to be invalid for RuO$_2$(110). The findings indicate that the stability of non-stoichiometric terminations is a more general phenomenon on transition metal oxide surfaces, similar to previous results for corundum-structured oxides. The study also discusses the importance of experimental preparation conditions, highlighting how the surface termination changes with oxygen pressure and its implications for catalytic reactions.