Photodissociation of ozone in the Chappuis band. I. Electronic structure calculations

Potential-energy surfaces of the 1 ¹A′, 1 ¹A″, and 2 ¹A″ states of ozone and corresponding transition-dipole-moment surfaces have been computed as a function of the two bond distances and the bond angle. The calculations are based on the complete-active-space self-consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) electronic-structure models. For the calculations of the ¹A″ surfaces, which exhibit a conical intersection, a diabatic representation has been constructed, employing a direct diabatization method implemented at the CASSCF level. The slow variation of the diabatic potentials and transition dipole moments with nuclear geometry allows us to perform the ab initio calculations on a widely spaced grid. The complete potential-energy and transition-dipole-moment surfaces are then efficiently obtained by interpolation. This procedure leads to very significant savings of computing time compared to the mapping of the rapidly varying potentials and derivative couplings in the conventional adiabatic representation. Diabatic potentials at the CASPT2 level have been obtained by applying the adiabatic-to-diabatic transformation constructed at the CASSCF level to the adiabatic CASPT2 potentials. The properties of the resulting adiabatic and diabatic ¹A″ potential-energy surfaces are discussed, with emphasis on the 1 ¹A″-2 ¹A″ conical intersection, which is of relevance for the photodissociation dynamics of ozone in the Chappuis band. The computation of the photoabsorption cross section and the comparison between theory and experiment are discussed in the accompanying paper.