Microscopic calculation of axial dispersion for quaterthiophene crystals

We present a microscopic calculation of the axial dispersion for the quaterthiophene crystal in a spectral region including the first and higher molecular excited states. After evaluating the complex dielectric tensor near the corresponding Frenkel excitons we observe that two of the principal axes referring to the upper Davydov states rotate with energy within the reflection plane of the monoclinic unit cell. In particular, we show the role of this dispersion in understanding the relationship between the transverse dielectric tensor and the measured optical spectra.