  We compare the performance of an approach using real frequency dependent
polarizability to compute optical absorption spectra to linear-response
time-dependent density functional theory (TD-DFT) for small organic dyes,
oligomers of different length (oligothiophenes), and molecular clusters
representing a molecular crystal (pentacene). For pentacene, the spectra
computed with the two methods are also compared to the spectrum computed for
clusters and the periodic solid using the dipole approximation. The approach
based on real polarizability produces spectra in good agreement with TD-DFT for
small molecules. The (artificial) redshift for longer oligomers is more
significant with the polarizability-based method than with TD-DFT. For
pentacene clusters, TD-DFT produces reasonable spectra with a hybrid
functional, but a significant redshift is introduced with a GGA functional due
to the presence of charge transfer transitions. This problem is slightly
attenuated with the polarizability-based method. The dipole approximation
results in spectra much redshifted vs both TD-DFT and the polarizability-based
method and in a different trend with cluster size.
