Two-dimensional H2O-Cl-2 and H2O-Br-2 potential surfaces: An ab initio study of ground and valence excited electronic states

International audience; All electron ab initio calculations for the interaction of H2O with Cl-2 and Br-2 are reported for the ground state and the lowest triplet and singlet Pi excited states as a function of both the X-X and O-X bond lengths (X = Cl or Br). For the ground state and lowest triplet state, the calculations are performed with the coupled cluster singles, doubles, and perturbative triple excitation level of correlation using an augmented triple-zeta basis set. For the (1)Pi state the multireference average quadratic coupled cluster technique was employed. For several points on the potential, the calculations were repeated with the augmented quadruple-zeta basis set. The ground-state well depths were found to be 917 and 1183 cm(-1) for Cl-2 and Br-2, respectively, with the triple-zeta basis set, and they increased to 982 and 1273 cm(-1) for the quadruple-zeta basis set. At the geometry of the ground-state minimum, the lowest energy state corresponding to the unperturbed (1)Pi states of the halogens increases in energy by 637 and 733 cm(-1), respectively, relative to the ground-state dissociation limit of the H2O-X-2 complex. Adding the attractive ground-state interaction energy to that of the repulsive excited state predicts a blue-shift, relative to that of the free halogen molecules, of approximate to 1600 cm(-1) for H2O-Cl-2 and approximate to 2000 cm(-1) for H2O-Br-2. These vertical blue-shifts for the dimers are greater than the shift of the band maximum upon solvation of either halogen in liquid water