The ultrafast nonradiative relaxation processes after photon excitation at 266 nm of Hg-2 trapped in a cryogenic argon matrix are studied by molecular 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 diatomics-in-molecules treatment of the mixing of the different states of Hg-2 induced by the Ar atoms. While the initially excited electronic state is the D1(u) state, we observe that after 10 ps of dynamics most of the electronic population (90%) is in the A0(g)(+/-) states. The majority of nonadiabatic jumps take place in the first 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 experimental steady-state spectroscopic studies. Finally, we also demonstrate the conservation of vibrational coherence of the Hg-2 stretching mode during the first few picoseconds, despite several nonadiabatic crossing events during this time.