  We perform ab initio calculations of optical properties for a typical
semiconductor conjugated polymer, poly-para-phenylenevinylene, in both isolated
chain and crystalline packing. In order to obtain results for excitonic
energies and real-space wavefunctions we explicitly include electron-hole
interaction within the density-matrix formalism. We find that the details of
crystalline arrangement crucially affect the optical properties, leading to a
richer exciton structure and opening non-radiative decay channels. This has
implications for the optical activity and optoelectronic applications of
polymer films.
