Excitons in conjugated polymers from first principles

By a combination of ab-initio computational techniques, based on density-functional theory, GW theory, and the Bethe–Salpeter equation, we study the opto-electronic properties of several conjugated polymers and in particular the properties of excitons. We study three different situations: (I) an isolated polymer chain, (II) a chain embedded in a dielectric medium, and (III) a polymer crystal. Surprisingly, the results obtained for situation (II) generally agree best with experiment. We discuss possible reasons for this rule and an interesting exception