Ab initio calculation of many-particle effects in the optical spectra of semiconductors

We present the first investigation of an optical spectrum including electron-hole pair interaction which starts from an ab initio calculation of Wannier functions and band structure. The resulting spectrum of TlCl agrees well with experiment, demonstrating the importance of both, Coulomb and self-consistently screened exchange interaction. Finally we present a Clausius-Mosotti-like formula with one fitting parameter which quantitatively takes into account the interaction effects.