Theoretical studies of neutral and charged water clusters

This article reviews recent theoretical work on (H₂O)_n, H⁺(H₂O)_n, and (H₂O)_n⁻ clusters. For the neutral clusters, considerable theoretical work has been carried out elucidating potential energy landscapes and how the complexity of the landscape impacts the thermodynamics and dynamics. Perhaps the most widely studied neutral cluster is the hexamer, which has several low-energy isomers of similar energy but with different H-bonding topologies. As a result, the hexamer has proven especially important for testing the performance of force fields and different electronic structure methods. Adding either an excess proton or electron to a neutral water cluster can lead to extensive changes in the cluster geometry. For the protonated clusters, much of the recent theoretical work has focused on interpreting the vibrational spectra which has proven especially challenging due to strong vibrational anharmonicities. In the case of the (H₂O)_n⁻ clusters, the emphasis has been on understanding the localization of the excess electron and its perturbation of the H-bonding network. Because the excess electron occupies a diffuse non-valence orbital, it is possible to describe these clusters using one-electron model Hamiltonians. High qualissty ab initio results are increasingly playing an important role in the parameterization of the model Hamiltonians.