The hydrated electron (e - (aq) ) 1 is perhaps the simplest quantum mechanical solute, consisting of a single excess electron embedded in liquid water. In addition to its role as a probe of condensed-phase dynamics, the excited states of e - (aq) have been implicated as reactive intermediates in the radiation damage of biomolecules such as DNA. Furthermore, e - (aq) is experiencing a renaissance of interest as a superreductant in photocatalysis and pollutant degradation. However, fundamental questions remain regarding the electronic and solvation structure of e - (aq) . There has thus been significant recent interest in testing theoretical models of e - (aq) by comparison to spectroscopic observables. 9-15 Our understanding of e - (aq) developed initially from oneelectron mixed quantum classical (1eMQC) simulations that showed e - (aq) carves out a solvent cavity and behaves roughly as a particle in a quasi-spherical box, with an s-like ground state. This cavity picture, supported by e - (aq) having a measured positive volume of solvation, has held up remarkably well in recent all-electron simulations, even though they employed substantially different electronic structure theories: self-interaction corrected density functional theory (DFT), Hartee-Fock, 21 MP2, and hybrid DFT. Although consensus has now been reached that the excess electron resides in a solvent cavity, questions remain over details of its electronic and solvation structure. How large is the solvent cavity? How much does the excess electron overlap with the solvating water molecules? Early models predicted an excess electron dominated by repulsion, leading to a highly-localized state with low overlap with the solvent and a first solvation shell comprising six water molecules. However, recent model-and all-electron simulations exhibit a large degree of overlap of the excess electron with the water. Only ∼0.5 e of charge is predicted to reside in the primary cavity, which has a four-fold coordination shell, with most of the remainder of the excess electron's charge overlapping the first and second solvent shells, in addition to a minor component encompassing a diffuse tail extending several Å from the cavity. This prompted Uhlig and co-workers to assign e - (aq) as having a complex nature, although it has been pointed out that spill out of the excess electron beyond its cavity is captured even by a simple particle-in-a-spherical-well model, with parameters chosen to reproduce the features of the measured optical and photoelectron spectra of e - (aq) . 27 As a consequence of its diffuse nature, the electron partially occupies the available frontier antibonding orbitals of the surrounding water molecules (σ * OH ), in a fashion similar to the charge-transfer associated with hydrogen-bond formation. Significant σ * OH occupations totalling ∼0.3 e had been predicted for surface-bound water cluster anions. We recently showed that these high antibonding occupations transfer to the condensed-phase e - (aq) . Quantifying the antibonding orbital occupation by e - is important to understanding how its dynamics are coupled to water motions. 1eMQC models, which neglect antibonding orbital occupation by construction, significantly underestimate the magnitude of coupling between the electron and water's intramolecular motions, compared to manyelectron simulations. It is worth noting that theories based on 1eMQC models are unable to rationalize the measured ultrafast excited-state lifetime of e - (aq) , which motivates a many-electron description.