Ab initio calculation of the electronic and optical excitations in polythiophene: Effects of intra- and interchain screening

We present an ab initio calculation of the electronic and optical excitatio ns of an isolated polythiophene chain as well as of bulk polythiophene. We use the GW approximation for the electronic self-energy and include exciton ic effects by solving the electron-hole Bethe-Salpeter equation. The inclus ion of interchain screening in the case of bulk polythiophene drastically r educes both the quasiparticle band gap and the exciton binding energies, bu t the optical gap is hardly affected. This finding is relevant for conjugat ed polymers in general.