d-electron frustration and the large fcc versus hcp binding preference in O adsorption on Pt(111)

First-principles calculations show that at saturation on Pt(111) terraces, O adatoms favor ``fcc'' over ``hcp'' sites by roughly 0.5 eV. This preference is a consequence of d-electron depletion between first- and second-neighbor Pt atoms in the fcc binding geometry, which does not occur in the hcp case. The source of the different d-electron distributions is the electrostatic requirement that the d hole on each Pt atom adjacent to an O lie along the line connecting the two.