Influence of structure on electron correlation effects and electron-water dispersion interactions in anionic water clusters

Electronic structure calculations at the level of second-order M?ller-Plesset perturbation theory have been performed on anionic water clusters, (H2O)n(-), in the n = 14-33 size regime. The contribution to the electron binding energy that arises from electron correlation is found to be significantly larger for cavity-bound electrons than it is for surface-bound electrons, even for surface states with electron binding energies well above 1 eV. A decomposition of the correlation energy into interactions between pairs of Boys-localized molecular orbitals is used to demonstrate that the larger correlation energy found in the cavity isomers arises from electron-water dispersion interactions, and that the dispersion interaction is larger in cavity-bound isomers because the unpaired electron penetrates well beyond the first solvation shell. In contrast, a surface-bound electron exhibits virtually no penetration into the interior of the cavity. To obtain a qualitatively accurate picture of this phenomenon, one must plot molecular orbitals using isoprobability surfaces rather than arbitrarily-selected isocontours.