Vibronic coupling in dimer - A convenient approximation revisited

The problem of linear vibronic coupling in model dimer is revisited with the objective of testing a version of the strong vibronic-coupling approximation that is recently used in crystal-optics applications. It is shown that in the limit of strong intermolecular resonance interaction (intense electronic transitions) this approximation fails in a spectacular way, predicting a set of artefact absorption bands. The consequences of this finding for the interpretation of the absorption spectra of oligothiophene crystals are discussed.