Detailed simulations are reported for the coupled dynamics of electrons and nuclei in the isomerization reaction of cis-stilbene stimulated by laser excitation. The results demonstrate that, in addition to the traditional vinyl and vinylphenyl torsions, the HCCH torsional coordinate of the vinyl group also makes a significant contribution to the HOMO and LUMO couplings that yield nonradiative electronic transitions to the ground state. Moreover, variations in the CCH angles of the vinyl group in the vicinity of the HOMO-LUMO couplings indicate that pyramidalization of the vinyl-group carbon atoms is strongly involved in these molecular orbital couplings. (C) 2003 Published by Elsevier B.V.