Solvatochromic Shifts of Uracil and Cytosine Using a Combined Multireference Configuration Interaction/Molecular Dynamics Approach and the Fragment Molecular Orbital Method

A recently developed combined quantum mechanics/molecular mechanics (QM/MM) approach has been applied to the calculation of solvatochromic shifts of the excited states of the pyrimidine nucleobases uracil and cytosine in aqueous solution. In this procedure the quantum mechanical solute is described using a multireference configuration interaction method while molecular dynamics simulations are used to obtain the structure of the solvent around the solute. The fragment molecular orbital multiconfiguration self-consistent field (FMO-MCSCF) method of Fedorov and Kitaura is also used and compared with the QM/MM results. The two methods give similar results. The solvatochromic shifts in uracil are found to be +0.41 (+0.44) eV for the S-1 excited state and -0.05 (-0.19) eV for the S-2 state at the QM/MM (FMO-MCSCF) level. Solvatochromic shifts in cytosine are calculated to be +0.25 (+0.19), +0.56 (+0.62), and +0.83 (+0.83) eV for the S-1, S-2, and S-3 states, respectively, at the QM/MM (FMO-MCSCF) level.