Linear absorption spectra of solvated thiouracils resolved at the hybrid RASPT2/MM level

On the example of 2-, 4- and 2,4-thiouracil we demonstrate the performance of the RASPT2/RASSCF protocol in reproducing the spectral positions and line shapes of linear absorption spectra that have been recorded in water and documented in this work. Through a QM/MM scheme coupled to a room-temperature Wigner sampling we simulate condensed phase spectra, permitting to compare our results against experiments. We discuss the sensitivity of the simulations to: a) the active space size by pushing the limits beyond the full-valence active spaces; b) the consideration of "dark" n pi*-states in the state averaging; c) the flavor of RASPT2 technique; d) the basis set The benchmarking demonstrates that full-pi valence active spaces tend to red-shift the absorption band. Increasing the active space rectifies the problem and we obtain near-quantitative agreement between our experiments and calculations. We, furthermore, demonstrate that the choice of RASPT2 flavor has to be made through rigorous benchmarking.