We present first principles molecular dynamics simulations of the photochemistry of butadiene and cyclohexadiene. The excited state is described with restricted open-shell Kohn-Sham theory. We observe cis-trans isomerizations for 1,3-butadiene and conrotatory ring opening of cyclohexadiene. The excitation of a sample of several butadiene molecules leads to the formation of an excimer. (C) 2003 American Institute of Physics.