THEORETICAL DETERMINATION OF THE ELECTRONIC-SPECTRUM OF FREE-BASE PORPHIN

The main features of the electronic spectrum of free base porphin are determined by means of multiconfigurational second-order perturbation (CASPT2) theory. The calculations comprise the optically allowed singl et states associated to the Q and B bands and the corresponding triple t states. The computed excitation energies deviate from the available experimental data by less than 0.3 eV in all the cases where a compari son is possible. It is found that both sigma- and pi-electron correlat ion contributions have to be taken into account in order to give a bal anced description of the ''cited states. A limited theoretical treatme nt, based on only pi-electron correlation or only partially including the correlation of the sigma electrons, is not sufficient.