  The computational simulation of photo-induced processes in large molecular
systems is a very challenging problem. Here, we present a detailed description
of our implementation of a molecular dynamics with electronic transitions
algorithm within the local-orbital density functional theory code Fireball,
suitable for the computational study of these problems. Our methodology enables
simulating photo-induced reaction mechanisms over hundreds of trajectories;
therefore, large statistically significant ensembles can be calculated to
accurately represent a reaction profile. As an example of the application of
this approach, we report results on the [2+2] cycloaddition of ethylene with
maleic anhydride and on the [2+2] photo-induced polymerization reaction of two
C60 molecules. We identify different deactivation channels of the initial
electron excitation, depending on the time of the electronic transition from
LUMO to HOMO, and the character of the HOMO after the transition.
