High level ab initio studies of the excited states of sulfuric acid and sulfur trioxide

We have calculated the vertical excitation energies and oscillator strengths of the lowest energy electronic transitions in sulfuric acid $(H2SO4)$ and sulfur trioxide $(SO3)$ with a range of ab initio methods. We have found that the highest level calculations with the complete active space self-consistent field and multireference configuration interaction (MRCI) methods predict transition energies much lower than those previously calculated with the simpler configuration interaction-singles method. The MRCI calculated electronic transitions for $SO3$ are in good agreement with the experimental results, whereas electronic transitions in vapor phase $H2SO4$ have yet to be observed. Our MRCI results suggest that the lowest lying electronic excitation in $H2SO4$ occurs around 144 nm and that the cross section in the actinic region is very small.