Effect of adsorbates on the isomer stability of Ir$_4$ clusters

The relative stability of Ir$_4$ isomers, both in the gas phase and on MgO(100) substrate, is studied using density functional theory. The square Ir$_4$ is the most stable in both cases. The metastable tetrahedral isomer, which experimental data suggest as the most stable form of Ir$_4$ on MgO(100), is highly distorted by the strong Ir--O interactions. The relative stability of Ir$_4$ isomers is strongly altered by adsorption of a single C atom since the binding energy of the C adatom to tetrahedral and butterfly Ir$_4$ is much larger ($\sim$ 1.7 eV) than that to the square one, both in the gas-phase and on MgO(100). After carbon adsorption, the most stable structure of Ir$_4$ is the butterfly geometry for free clusters and the ``tetrahedral'' one for Ir$_4$/MgO(100). The C adatom binds in a bridge configuration in all cases and reduces the distortions produced by the MgO substrate. Energetics and equilibrium atomic geometries will also be discussed for the adsorption ({\it i}) one H or O atom and ({\it ii}) one CO molecule.