The ozone ring closure as a test for multi-state multi-configurational second order perturbation theory (MS-CASPT2)

The open and closed forms of ozone and the path connecting them through a transition state, in C2v symmetry, are explored using the multi-state, multi-configurational second order perturbative method, MS-CASPT2. It is demonstrated that, by using an ANO-L triple-ζ basis set, it is possible to set up an active space able to describe the otherwise troublesome transition state region. Both a conical intersection and a near degeneracy region between the 11A1 and 21A1 states are located in the vicinity of the transition state. The relative position of the intersection and the transition state are discussed.