A theoretical study of the electronic spectrum of water

The electronic spectrum of water is investigated using coupled cluster electronic structure methods. Vertical excitation energies are calculated for both gas phase H2O, various water clusters, as well as from models designed to simulate condensed phase effects. Four different approaches for describing condensed phase effects on the electronic transitions are investigated: continuum (a single water molecule embedded in a dielectric medium), discrete (water clusters), semidiscrete (a water pentamer cluster embedded in a dielectric medium), and intermolecular perturbation methods. The results are compared with experimental results. The solvent shift on the lowest state appears to be reasonably described by discrete and semidiscrete methods. It is very difficult to model the condensed-phase effects for the higher states of the pure liquid. (C) 2000 American Institute of Physics. [S0021-9606(00)30942-4].