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

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.