  First principles methods based on periodic boundary conditions are used
extensively by materials theorists. However, applying these methods to systems
with confined electronic states entails the use of large unit cells in order to
avoid artificial image interactions. We present a general approach for
truncating the Coulomb interaction that removes image effects directly and
leads to well converged results for modest-sized periodic cells. As an
illustration, we find the lowest-energy quasiparticle and exciton states in
two-dimensional hexagonal GaN sheets. These sheets have been proposed as parent
materials for single-walled GaN nanotubes which may be of interest for
optoelectronics.

Materials Science;Soft Condensed Matter