Computer Simulation of Photochemical Reactions Yusheng Dou, Department of Physical Sciences, Nicholls State University, Thibodaux, LA 70301 Nonadiabatic transition for * and n* excitations in trans-cis isomerization of azobenzene For * excitation, the couplings between the HOMO and LUMO, which lead to the decay of the excited molecule to electronic ground state, occur at the times when the molecule finds a partially twisted structure and few twisted geometries. The former channel leads to the formation of the trans isomer and the latter ones guide the reaction toward either the trans or cis isomer. For n* excitation, the couplings between the HOMO and LUMO occurs at the time when the molecule finds a geometry, which leads to the formation of either the trans or cis isomer. The opening of the decay channel at the partially twisted structure for * excitation accounts for smaller quantum yield of the isomerization.