  We use ab initio density functional calculations to study the effect of
electron and hole doping on the equilibrium geometry and electronic structure
of C, Si, and S monatomic wires. Independent of doping, all these nanowires are
found to be metallic. In absence of doping, C wires are straight, whereas Si
and S wires display a zigzag structure. Besides two preferred bond angles of 60
deg and 120 deg in Si wires, we find an additional metastable bond angle of 90
deg in S wires. The equilibrium geometry and electronic structure of these
nanowires is shown to change drastically upon electron and hole doping.
