Does density functional theory contribute to the understanding of excited states of unsaturated organic compounds?

A comparative study has been performed on the electronic spectra of a numbe r of unsaturated organic molecules, using on the one hand density functiona l linear response theory and on the other multiconfigurational second-order perturbation theory, in order to establish the accuracy that the density f unctional based methods can give For excitation energies and energy surface s for excited states. The following molecules are included in the study: te trazine; the five-membered ring systems cyclopentadiene, furan, pyrrole, an d thiophene; acetone; and a dipeptide. The results show that DFT valence ex cited states have errors that vary between 0 and 1 eV, while Rydberg states are accurate to about 0.2 eV in most cases. The use of an asymptotically c orrected exchange-correlation potential was essential for the latter result . However, transitions which involve a considerable charge transfer have mu ch larger errors. The results show in some cases a surprisingly strong inte raction between valence and Rydberg excited states, with considerable effec ts both on energies and computed intensities.