MULTICONFIGURATIONAL PERTURBATION THEORY APPLIED TO EXCITED STATES OF ORGANIC COMPOUNDS

Applications of the Complete Active Space (CAS) SCF method in conjunction with multiconfigurational second-order perturbation theory (CASPT2) in electronic spectroscopy of organic molecules are reviewed. Since the first applications in spectroscopy were performed at the beginning of the present decade, the CASSCF/CASPT2 method has been used to study electronic spectra of a large number of compounds. The experience gained from this global investigation is illustrated in the present contribution through several examples. In most cases, the CASSCF reference function does characterize with sufficient accuracy the states of interest, which supports the use of a single reference perturbation theory in spectroscopic studies of organic systems. The CASSCF/CASPT2 method is capable of yielding accurate results for relative energies and other properties of excited states, provided that flexible one-electron basis sets are used and an appropriate active space can be chosen. The overall accuracy of the approach is high. The excitation energies are usually found to be within ±0.2 eV of the available experimental energies for correctly assigned transitions. The review covers some of the most recent applications in the spectroscopy of organic compounds: absorption spectrum of free base porphin employing an extended treatment; vertical, nonvertical, and emission energies of long polyenals; spectra of trans- and cis-stilbene together with an analysis of certain static aspects of the photo-induced isomerization process; absorption spectra of purine DNA base monomers and related compounds, and analysis of the spectroscopic features of polypeptides based on intra- and interpeptide charge transfer transitions. In addition, the electronic spectra of organic compounds with interacting double bonds are rationalized. These studies either confirm existing experimental assignments or lead to new predictions and a novel understanding of the electronic spectra of the corresponding organic molecules.