Comparison of optical absorption spectra of organic molecules and aggregates computed from real frequency dependent polarizability to TD-DFT and the dipole approximation.

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 lengths (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 slightly 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 generalized gradient approximation 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.