Theoretical investigation of absorption spectrum of SO2 molecule: Including S1–S2 vibronic coupling

In this work, we present a model Hamiltonian including three vibrational modes of sulfur dioxide molecule and determine the potential energy surfaces of the lowest two excited states S1[1A2] and S2[1B1], which are strongly coupled to each other. The leading coefficients of the Taylor expansion of the diabatic potential-energy surfaces in terms of ground-state normal coordinates at the reference geometry have been obtained at the CIS/6-311++G(3df, 3pd) level. The gas phase absorption spectrum of SO2 molecule has been calculated in the 2400–3400 Å region using the time correlation theory, including vibronic coupling. The absorption spectrum is in acceptable agreement with the experimental spectrum.