It has often been taken for granted that rare-gas adsorbates on metal surfaces would prefer maximally coordinated hollow sites. Our calculation show, however, that the hitherto accepted picture is incorrect: Xe in the square-root-3 structure on Cu(111), Pd(111), Pt(111), Ti(0001), Ag(111), and Mg(0001) assumes the on-top position. The very small adsorption energy makes the analysis particularly demanding. We identify the importance of polarization and a site-dependent Pauli repulsion in actuating the site preference and the principle nature of the rare-gas atom--metal surface interaction. We anticipate that these findings may also be generally valid for other rare-gas/metal systems, thus presenting a new picture of rare-gas adsorption, where the on-top site is the rule rather than the exception. [J. L.F. Da Silva, C. Stampfl, and M. Scheffler, Phys. Rev. Lett. 90, 066104 (2003).]

Old picture: Rare-gas atoms in hollow sites (left)

New picture: Rare-gas atoms in on-top sites (right)