The photodissociation and photoisomerization of ICN in water is studied using molecular dynamics simulations. A water-ICN potential energy function that takes into account the different ground and excited state charges and their shift as a function of the reaction coordinate is developed. The calculations include nonadiabatic transitions between the different electronic states and allow for a complete description of the photodissociation leading to ground-state and excited-state iodine and to recombination producing ICN and INC. The calculated UV absorption spectrum, the cage escape probability, the quantum yield of ICN and INC, and the subsequent vibrational relaxation rate of ICN and INC are in reasonable agreement with recent experiments. The trajectories provide a detailed microscopic picture of the early events. For example, it is shown that most recombination events on the ground state involve nonadiabatic transitions before the molecule has a chance to completely dissociate on the excited state, and that the quantum yield for photoisomerization to form INC is statistically determined very early in the photodissociation process. (C) 2003 American Institute of Physics.