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Self-consistent calculations show that S and Al adatoms form directional bonds to Al(001) in four-fold hollows. If substrate nearest-neighbor nuclei are allowed to move, each adatom approaches the surface, as its square of neighbors expands and moves slightly into the metal. These relaxations approximately preserve adatom-Al bond lengths. They open Al-adatom-Al bond angles slightly, and shorten the bonds between first- and second-neighbor atoms in the outer Al layer, as expected for a surface under tensile stress.
Peter J. Feibelman (1989) studied this question.