A method to locate minimum energy conical intersection (MECI) points with the analytical derivative technique for multistate complete active space second-order perturbation theory is presented. The photochemical ring-opening reaction of cyclohexadiene (CHD) is revisited to examine the effect of dynamic electron correlation on MECI properties. The geometries as well as potential energy landscapes of CHD are found to change remarkably. The energy difference gradient (g) and interstate coupling (h) vectors consisting the branching plane of MECI are also strongly affected. These results indicate the importance of accurate potential energy surfaces for fully understanding photochemical reactions. (C) 2009 Elsevier B.V. All rights reserved.