Magic structures of H-passivated $\langle110 \rangle$ silicon nanowires

We report a genetic algorithm approach combined with ab initio calculations to determine the structure of hydrogenated $\langle 110\rangle$ Si nanowires. As the number of atoms per length increases, we find that the cross section of the nanowire evolves from chains of six-atom rings, to fused pairs of such chains, to hexagons bounded by \{001\} and \{111\} facets. Our calculations predict that hexagonal wires become stable starting at about 1.2 nm diameter, which is consistent with recent experimental reports of nanowires with diameters of about 3 nm.