  Using a first-principle approach beyond density functional theory we
calculate the electronic and optical properties of small diameter CdSe
nanowires.Our results demonstrate how some approximations commonly used in bulk
systems fail at this nano-scale level and how indispensable it is to include
crystal local fields and excitonic effects to predict the unique optical
properties of nanowires. From our results, we then construct a simple model
that describes the optical gap as a function of the diameter of the wire, that
turns out to be in excellent agreement with experiments for intermediate and
large diameters.
