  In this paper we present a detailed analysis of the atomic and electronic
structure of GaAs nanowires using first-principles pseudopotential
calculations. We consider six different types of nanowires with different
diameters all grown along [111] direction and reveal interesting trends between
cohesive energy and nanowire type with varying diameters. Generally, the
average cohesive energy of nanowires with wurtzite stacking is higher than
those with zinc blende stacking for small diameters. We found that most of the
bare nanowires considered here are semiconducting and continue to be
semiconducting upon the passivation of surface dangling bonds with hydrogen
atom. However, the surface states associated with the surface atoms having two
dangling bonds in zinc blende stacking occur in the band gap and can decrease
the band gap to change the nanowire from semiconducting to metallic state.
These nanowires become semiconducting upon hydrogen passivation. Even if the
band gap of some nanowires decreases with increasing diameter and hence reveals
the quantum confinement effect, generally the band gap variation is rather
complex and depends on the type and geometry, diameter, type of relaxation and
also whether the dangling bonds of surface atoms are saturated with hydrogen.
