Does the hydrated electron occupy a cavity in liquid water
In plain words
An extra electron dissolved in water forms the hydrated electron, a key species in radiation chemistry. Whether it is held in a small empty pocket among water molecules or overlaps a region where water molecules crowd in is disputed.
Precise statement
For the excess electron in liquid water at 300 K, determine whether its ground-state density occupies a solvent cavity of radius about 2.5 Angstrom with depleted water density, or overlaps a region of enhanced water density (the non-cavity model of 2010), by reproducing the absorption band (peak near 1.7 eV with its high-energy tail), the radius of gyration (about 2.5 Angstrom), the resonance Raman spectrum and the partial molar volume. An answer is an ab initio simulation with quantum nuclei that reproduces these observables and fixes the structure.
What would settle it
Path-integral ab initio simulations with hybrid or better functionals, or correlated wavefunction methods, compared with absorption, resonance Raman and scattering data.
Status in the literature
Most ab initio simulations favor a cavity, but sensitivity to the functional has kept the 2010 non-cavity proposal (Larsen, Glover, Schwartz, Science, https://doi.org/10.1126/science.1189588) under discussion.