Simulations of the photoconversion of 4a,4b-dihydrophenanthrene (DHP) to stilbene, using semiclassical electron-radiation-ion dynamics, are reported. The carbon single bond connecting the phenyl rings breaks approximately 200 fs after the laser pulse is turned off. In the results shown here, the excited cis-stilbene molecule then further isomerizes to the trans conformation after about 1 ps. Each of these two changes of molecular geometry involves a HOMO-LUMO avoided crossing, which produces a transfer of electronic population from LUMO to HOMO. These detailed dynamical simulations are consistent with the interpretation that ring-opening in stilbene is a single photon process. (c) 2007 Elsevier B.V. All rights reserved.