The ultrafast nonradiative relaxation processes after photon excitation at 266 nm of Hg2 trapped in a cryogenic Ar matrix were studied by mol. dynamics with quantum transitions. Fourteen electronic surfaces and their corresponding couplings are included in the simulations. The couplings between electronic states are modeled using a DIM treatment of the mixing of the different states of Hg2 induced by the Ar atoms. While the initially excited electronic state is the D1u state, after 10 ps of dynamics most of the electronic population (90%) is in the A0g+- states. The majority of nonadiabatic jumps take place in the 1st 2 ps of the dynamics and at large Hg-Hg nuclear distances close to the Franck-Condon region of excitation. These results confirm predictions from previous exptl. steady-state spectroscopic studies. Finally, the authors also demonstrate the conservation of vibrational coherence of the Hg2 stretching mode during the 1st few picoseconds, despite several nonadiabatic crossing events during this time. [on SciFinder (R)]