Ab initio complete active space self-consistent field calculations and the second-order Moller-Plesset calculations have been performed to examine the photoisomerization from trans-4-dimethylamino, 4'-cyanostilbene (DCS). Firstly, the conical intersection (CIX), where the radiationless relaxation from S-1 into S-0 takes place, was located. The S-1/S0CIX of DCS is a crossing region between zwitterionic and covalent diradical states, which is also found in stilbene and styrene. Then the potential energy surfaces with respect to the central enthylenic torsional coordinate were calculated to discuss the photoisomerization from trans-DCS to the S-1/S-0-CIX. (C) 2003 Elsevier Science B.V. All rights reserved.