We investigate the role of surface stress in the hex reconstruction of (0 0
1) transition-metal surfaces introducing the density-functional-theory str
ess density, a theoretical tool allowing the local analysis of the stress.
By ab initio total energy, stress, and stress density calculations for Ir (
0 0 1) we show that the reconstruction is due to the concurrence of tensile
stress relief in the surface layer and the huge outward relaxation thereof
The latter causes a compressive stress relief in the region between the su
rface layer and the substrate, leading to an increased total surface stress
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