Ab initio calculation of the quasiparticle spectrum and excitonic effects inLi2O

We report an ab initio calculation of the binding energies and the nature of the excitonic states in the near-gap absorption spectrum of a real solid, ${\mathrm{Li}}_2$O. We calculate the ground-state properties using density-functional theory together with soft pseudopotentials. Applying Hedin's GW approximation for the self-energy corrections to the band structure, we determine the minimal gap about 1 eV above the measured absorption onset. Finally, we obtain agreement with experiment by solving an effective two-particle Schrödinger equation for the electron-hole pairs.