  Using a simple atomistic model of anharmonic nearest-neighbors interaction,
we have calculated the step energies of strained hexagonal monolayer islands.
These have been found to decrease with the absolute value of the misfit due to
the strain relaxation at steps. The effect is significantly more pronounced in
the case of positive misfit owing to the stronger repulsive interatomic forces.
Furthermore, (111)-faceted steps are favored at positive misfit (compressed
islands) and, to a lesser extent, (100)-faceted steps at negative misfits
(tensile islands). The result is rationalized in terms of the different bonding
geometries at step edges and a comparison with experiments is included. Thus,
the equilibrium shape transforms from regular hexagons at zero misfit to
threefold symmetric hexagons with increasing misfit.
