  While, the sulfur dioxide molecule has been extensively studied over the last
decades, its photoexcitation dynamics is still unclear, due to its complexity,
combining conical intersections and spin-orbit coupling between a manifold of
states. We present for the first time a comprehensive ab initio study of the
intersystem crossing of the molecule in the low energy domain, based on a
wave-packet propagation on the manifold of the lowest singlet and triplet
states. Furthermore, spin-orbit couplings are evaluated on a geometry-dependent
grid, and diabatized along with the different conical intersections. Our
results show for the first time the primordial role of the triplet $^3$B$_2$
state which has not been discussed before, giving new insights into the
dynamics of the intersystem crossing in $SO_2$.
