Accurate characterization of the lowest triplet potential energy surface of SO ₂ with a coupled cluster method

The near-equilibrium potential energy surface (PES) of the ã 3B1 state of SO ₂ is developed from explicitly correlated spin-unrestricted coupled cluster calculations with single, double, and perturbative triple excitations with an augmented triple-zeta correlation-consistent basis set. The lowest-lying ro-vibrational energy levels of several sulfur isotopologues have been determined using this PES. It is shown that the new ab initio PES provides a much better description of the low-lying vibrational states than a previous PES determined at the multi-reference configuration interaction level. In particular, the theory-experiment agreement for the three lowest-lying vibrational transitions is within 1-3 cm ⁻¹ .