A dynamical picture and the detailed changes in the structure for the ring opening reaction of cyclohexadiene (CHD) to form hexatriene (HT) are studied by a semiclassical electron radiation-ion dynamics (SERID) simulation. Following the simulated trajectory, the potential energy curves (PECs) of three lowest singlet states (S-0, S-1 and S-2) have been calculated at the CASSCF/MRPT2 level with 6-31G* basis sets. Two non-adiabatic state-to-state transitions are found at 112 fs and 129 fs, which confirms the mechanism suggested in the Ref. [M. Garavelli, C. S. Page, P. Celani, M. Olivucci, W. E. Schmid, S. A. Trushin, W. Fuss, J. Phys. Chem. A 105 (2001) 4458]. (C) 2009 Elsevier B. V. All rights reserved.