  The reconstruction mechanism of (001) fcc transition metal surfaces is
investigated using a full-potential all-electron electronic structure method
within density-functional theory. Total-energy supercell calculations confirm
the experimental finding that a close-packed quasi-hexagonal overlayer
reconstruction is possible for the late 5$d$-metals Ir, Pt, and Au, while it is
disfavoured in the isovalent 4$d$ metals (Rh, Pd, Ag). The reconstructive
behaviour is driven by the tensile surface stress of the unreconstructed
surfaces; the stress is significantly larger in the 5$d$ metals than in 4$d$
ones, and only in the former case it overcomes the substrate resistance to the
required geometric rearrangement. It is shown that the surface stress for these
systems is due to $d$ charge depletion from the surface layer, and that the
cause of the 4th-to-5th row stress difference is the importance of relativistic
effects in the 5$d$ series.
