  Absorption spectra of small molecular aggregates (oligomers) are considered.
The dipole-dipole interaction between the monomers leads to shifts of the
oligomer spectra with respect to the monomer absorption. The line-shapes of
monomer as well as oligomer absorption depend strongly on the coupling to
vibrational modes. Using a recently developed approach [Roden et. al, PRL 103,
058301] we investigate the length dependence of spectra of one-dimensional
aggregates for various values of the interaction strength between the monomers.
It is demonstrated, that the present approach is well suited to describe the
occurrence of the J- and H-bands.
