On the electronically excited states of uracil
Vertical excitation energies in uracil in the gas phase and in water solution are investigated by the equation-of-motion coupled-cluster and multi-reference configuration interaction methods. Basis set effects are found to be important for converged results. The analysis of electronic wave functions reveals that the lowest singlet states are predominantly of a singly excited character and are therefore well described by single-reference equation-of-motion methods augmented by a perturbative triples correction to account for dynamical correlation. Our best estimates for the vertical excitation energies for the lowest singlet n->pi* and pi->pi* are 5.0+/-0.1 eV and 5.3+/-0.1 eV, respectively. The solvent effects for these states are estimated to be +0.5 eV and +/-0.1 eV, respectively. We attribute the difference between the computed vertical excitations and the maximum of the experimental absorption to strong vibronic interaction between the lowest A'' and A' states leading to intensity borrowing by the forbidden transition.