Effective charges and the surface stability of O on Cu(001)

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Abstract
The effective charge on a surface atom gives both the force on the atom in terms of an external, unscreened field, and the change in the surface dipole contribution to the work function when the atom is displaced. It is shown that this can give rise to a surface instability if the effective charge is big enough. In the case of O adsorbed on Cu(001) in the fourfold hollow site, calculations give an effective charge of -0.9‖e‖, which means that the ‘‘ideal’’ c(2×2) structure at 0.5-monolayer coverage is unstable. This provides an explanation for the reconstruction to the (√2 ×2 √2 )R45° phase which is observed.