The interpretation of spectroscopic results requires the application of modern high-quality theoretical methods. Experimentally determined excitation energies, ionization potentials, vibrations, vibrational couplings or intensities of vibrational transitions can be usually only interpreted if a comparison with theoretical predictions takes place. Ab initio and DFT or TD-DFT calculations are performed on a level appropriate for the size of the system and the question of the investigated problem. Commercially available programs are used such as Gaussian, Turbomole or Molcas; the latter was developed particularly for CASSCF and CASPT2 calculations. In the case of aromatic systems, these methods are particularly suitable for the prediction of electronic excitation energies and ionization potentials as well as for the computation of reaction coordinates. The harmonic (scaled) vibrations which are obtained from quantum chemical calculations are very helpful for an interpretation of the vibrational spectra of neutral or ionic species. However, in the case of hydrogen-bonded clusters both anharmonic effects and couplings between different vibrations are frequently observed. Program packages have been developed in order to analyze the multi-dimensional vibrational couplings in clusters. Programs for the computation of multi-dimensional Franck-Condon integrals have been implemented in order to be additionally able to describe the intensities of the spectra.
