The overall energetics and the feature of reactive potential energy surfaces for the photochemical interconversion between cyclohexadiene (CHD) and all-cis-hexatriene (cZc-HT) have been investigated using the multireference configuration interaction (MRCI) calculations. The adiabatic and the diabatic potential energy surfaces of the ground and the excited states have been calculated along the Jacobi coordinates. The conical intersections among the states are estimated and the corresponding non-adiabatic transition probabilities are calculated using the semiclassical Zhu-Nakamura formula. The 1(1)B-2(1)A decay occurs by C-2-symmetry-breaking motion around the conical intersection. The non-adiabatic transition to 1(1)A occurs by; the motion toward the 5-membered ring. (C) 2004 Elsevier B.V. All rights reserved.