Computing optical absorption spectra from first principles: Self-energy and electron-hole interaction effects

A method for the inclusion of self-energy and excitonic effects in first-principles calculations of absorption spectra, within the state-of-the-art plane-wave pseudopotential approach, is discussed. Self-energy effects are computed within GW; and the electron-hole interaction is treated solving an effective tyro-particle equation which is derived from the relevant Bethe-Salpeter equation. We review numerical results for three systems: a small sodium cluster, the lithium oxyde insulating crystal, and bulk silicon, the prototype semiconductor. In the case of silicon, we present new results obtained considering additional approximations intended to reduce the computational effort and generally employed in Wannier-Mott exciton calculations, and discuss their reliability.