Ab initio determination of potential energy surfaces for the first two UV absorption bands of SO2.

Three-dimensional potential energy surfaces for the two lowest singlet (Ã(1)B1 and B̃(1)A2) and two lowest triplet (ã(3)B1 and b̃(3)A2) states of SO2 have been determined at the Davidson corrected internally contracted multi-reference configuration interaction level with the augmented correlation-consistent polarized triple-zeta basis set (icMRCI+Q∕AVTZ). The non-adiabatically coupled singlet states, which are responsible for the complex Clements bands of the B band, are expressed in a 2 × 2 quasi-diabatic representation. The triplet state potential energy surfaces, which are responsible for the weak A band, were constructed in the adiabatic representation. The absorption spectrum spanning both the A and B bands, which is calculated with a three-state non-adiabatic coupled Hamiltonian, is in good agreement with experiment, thus validating the potential energy surfaces and their couplings.