lap with first-shell coordinating water molecules. Using NBO analysis, we provided a quantitative rationale for this complex behavior: the excess electron has ∼0.3 e occupation of σ * OH orbitals in total brought about by large electronic couplings of a cavity state and the antibonding orbitals. This leads to an enhanced dynamical coupling between the excess electron's energy levels and water stretch vibrations compared to a MQC model. Furthermore, the weakening of OH bonds by σ * OH occupation leads to stretch and bend vibrations that are redshifted compared to pure water, in quantitative agreement with experiment. Somewhat surprisingly, the σ * OH occupation of a single water molecule can reach 0.2 e, which leads to a noticeable extension of that OH bond and a reduction of its antibonding orbital energy by ∼1 eV.