Molecular Precursors in the Dissociative Adsorption of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>on Pt(111)

Ab initio local-spin-density calculations for the adsorption of ${\mathrm{O}}_{2}$ on Pt(111) are presented. We identify two distinct, but energetically almost degenerate chemisorbed precursors. A superoxolike paramagnetic precursor is formed at the bridge site, with the molecule parallel to the surface. A second peroxolike nonmagnetic precursor is formed in the threefold hollow, with the atom slightly canted in a top-hollow-bridge geometry. The nature of the barrier for dissociation into atoms adsorbed in the hollows is explored.