  The adsorption of CO on unreconstructed and reconstructed Ir{100} has been
studied, using a combination of density functional theory and thermodynamics,
to determine the relative stability of the two phases as a function of CO
coverage, temperature and pressure. We obtain good agreement with
experimentaldata. At zero temperature, the (1X5) reconstruction becomes less
stable than the unreconstructed (1X1) surface when the CO coverage exceeds a
critical value of 0.09 ML. The interaction between CO molecules is found to be
repulsive on the reconstructed surface, but attractive on the unreconstructed,
explaining the experimental observation of high CO coverage on growing (1X1)
islands. At all temperatures and pressures, we find only two possible stable
states: 0.05 ML CO c(2X2) overlayer on the (1X1) substrate, and the clean
(1$\times$5) reconstructed surface.
