Abstract
A first principles method will be presented for the calculation of quasiparticle eigenstates in a polymer crystal strongly interacting with electromagnetic radiation. The matter field will be represented by delocalized excitons whose energy spectrum and wavefunction will be determined with the help of the ab initio Green’s function formalism of charge transfer exciton theory. The method has been applied for the computation of bulk polariton bands in highly conducting polydiacetylene crystals.