  Since it has been shown that the silicon trimer has a highly multi-reference
character, accurate multi-reference configuration interaction calculations are
performed to elucidate its electronic manifold. Emphasis is given to the long
range part of the potential, aiming to understand the atom-diatom collisions
dynamical aspects, to describe conical intersections and important saddle
points along the reactive path. Potential energy surface main features analysis
are performed for benchmarking, and highly accurate values for structures,
vibrational constants and energy gaps are reported, as well as the unpublished
spin-orbit coupling magnitude. The results predict that inter-system crossings
will play an important role in dynamical simulations, specially in triplet
state quenching, making the problem of constructing a precise potential energy
surface more complicated and multi-layer dependent. The ground state is
predicted to be the singlet one, but since the singlet-triplet gap is rather
small (2.448 kJ/mol) both of them are expected to be populated at room
temperature.
