Combining ab initio multiconfigurational and Free Energy Gradient methods to study the π–π* excited state structure and properties of uracil in water

The Free Energy Gradient method is used to obtain the π–π* excited state structure of uracil in aqueous environment. The geometry calculations are made at the CASSCF level. The CASPT2 method is employed to calculate absorption and emission energies of uracil in gas and in aqueous solution. An average discrete electrostatic model is used to include the solvent effect. The results for the calculated absorption and emission transitions are found in good agreement with experiment both in gas phase and in water. The solvent effect on the Stokes shift is also calculated in good agreement with experiment. These agreements lend additional credence to the structures obtained for the lowest π–π* excited state.