Having found that the theoretical XAS of e - (aq) is in quantitative agreement with experiment, both in terms of the energy of the 533 eV peak and its intensity, we turn now to the assignment of this feature. Representative natural transition orbitals (NTO) 42 for a 533 eV excitation are shown in Fig. , where the hole (h) NTO (green surface) is a localized O1s core orbital as expected. The particle (p) NTO (blue and red surfaces), on the other hand, is partially localized in an s-like state of the cavity of e - (aq) while also being delocalized over multiple first-and second-shell water molecules, in a manner very reminiscent of the singly occupied molecular orbital (SOMO) from many-electron simulations (see e.g. Fig. of Ref. 24). The similarity of the p to the SOMO can be quantified by computing their spatial overlaps, S = ⟨ϕ SOMO |ϕ p-NTO ⟩ . For transitions below 533.9 eV, we find the average oscillator-strength-weighted overlap is 0.80 (see SI for definition), indicating a high degree of similarity. In other words, we can think of the 533 eV transition as resulting from O1s excitations to the half-filled excess-electron's orbital.