  Water is often viewed as a collection of monomers interacting
electrostatically with each other. We compare the water proton momentum
distributions from recent neutron scattering data with those calculated from
two electronic structure based models. We find that below 500 K the
electrostatic models are not able to even qualitatively account for the sizable
vibrational zero-point contribution to the enthalpy of vaporization. This
discrepancy is evidence that the change in the proton well upon solvation
cannot be entirely explained by electrostatic effects alone.
