[en] We report ab initio investigations of hexagon-shaped [111]/[0001] oriented III-V semiconductor nanowires with varying crystal structure, varying surface passivation, and varying diameter. Their stability is dominated by the free surface energies of the corresponding facets which differ only weakly from those of free surfaces. We observe a phase transition between local zinc-blende and wurtzite geometry of the rods versus the preparation conditions of the surfaces, which is accompanied by a change in the facet orientation