A theoretical study of the low-lying excited states of ozone

A theoretical study has been performed on the five lowest excited states of the ozone molecule using multiconfigurational second-order perturbation theory (CASPT2). The predicted order of states is: 3A2 (T0 = 1.15 eV), 3B2 (T0 = 1.33 eV, 3B1 (T0 = 1.33 eV), 1A2 (T0 = 1.44 eV) and 1B1 (T0 = 1.88 eV). Corresponding experimental data are: 1.18, 1.30, 1.45, 1.58, and 2.05 eV, respectively. Equilibrium geometries, harmonic frequencies of symmetric vibrations, and vertical excitation energies are also reported. The dissociation limit De for the ground state of ozone is found to be 1.08 eV, in agreement with the experimental value (1.13 eV). The calculations make use of a modified Fock operator in the CASPT2 theory. Relative energies of states with a different number of open shells are substantially improved. The modified CASPT2 method was checked by calculating spectroscopic constants of the oxygen molecule.