Excitons and optical properties of alpha-quartz

We present an ail initio study of the optical properties of ru-quartz. The absorption spectrum is calculated by solving the Bethe-Salpeter equation fo r the interacting electron-hole system and found to be in excellent agreeme nt with the measured spectrum up to 10 eV above the absorption threshold. W e find that excitonic effects are crucial in understanding the sharp featur es in the absorption spectrum in this energy range. They are also crucial i n the nb initio computation of the static dielectric constant, significantl y enhancing its value.