Time-dependent density functional theory for polycyclic aromatic hydrocarbon anions: What is the best approach

The electronic spectra of a variety of large polycyclic aromatic hydrocarbon (PAH) anions are required to assess their importance in the interstellar medium. Using the 6-31G** basis set with a set of diffuse pπ functions on the carbon atoms removes all of the unwanted σ* solutions found using the 6-31++G** basis set. The unwanted π* solutions can be removed in the normal way, however, for the PAH anions, it appears that using only the 6-31G** basis set offers a cost effective way of determining the spectra of astrophysical interest. A comparison of the computed and experimental results supports this suggestion.