The open and closed forms of ozone and the path connecting them through a transition state, in C-2v 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-zeta 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 1(1)A(1) and 2(1)A(1) states are located in the vicinity of the transition state. The relative position of the intersection and the transition state are discussed. (c) 2008 Elsevier B. V. All rights reserved.