A b i n i t i o calculation of the first order interaction energy in excited dimers. The H2O–H2O and H2O–Ne dimers

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Abstract
In order to interpret the solvation blue shift in the 1A1→1B1 UV band in H2O (observed in liquid water, ice, and H2O embedded in rare gas matrices) we have made ab initio calculations of the first order electrostatic and exchange interaction energies in H2O–H2O and H2O*–Ne dimers, after extending the usual symmetry adapted perturbation expressions to excited state molecules.We have found this blue shift to be caused mainly by the enlarged exchange repulsion between the excited H2O molecule and its neighbors, originating from the extended (Rydberg) character of the excited 1B1 state. The orientational dependence of this exchange repulsion has been calculated and correlated with the spatial distribution of the Rydberg state. The transition‐dipole resonance interaction was found to be of little importance. These results were confirmed by supermolecule MO calculations on the ground state and excited dimers which showed moreover, an enlarged polarization of the excited H2O ( compared with the ground state), as well as some other effects that may be artifacts from the supermolecule treatment of excited dimers.