  We present an ab initio exploration of the phenomena which will become
important for freestanding structures of silicon as they are realized on the
nanoscale. We find that not only surface but also edge effects are important
considerations in structures of dimensions on the order of 3 nm. Specifically,
for long nanoscale silicon bars, we find two competing low-energy
reconstructions with a transition from one to the other as the cross section of
the bar decreases. We predict that this size-dependent phase transition has a
signature in the electronic structure of the bar but little effect on elastic
properties.
