Effect of overscreening on the localization of hydrated electrons

The problem of the ground state of hydrated electron is revisited with a fo cus on the effects due to nonlocal dielectric response of water. The standa rd variational analysis is performed. It takes into account - in addition t o nonlocal polarization of nuclear modes - the electron repulsion from the closed shells of water molecules, a possibility to form a cavity around the electron and the interaction of the hydrated electron with the high freque ncy electronic degrees of freedom of water. The classical dielectric contin uum limit, shown for a reference, gives the ground state hydration energy t hat is 2.5 times smaller than the experimental value. The situation alters dramatically if one accounts for the nonlocal dielectric response of water. If one takes literally the existing MD simulation data for the static wave -vector dependent dielectric response function (with an "overscreening" res onance at k similar to 3 Angstrom (-1)), the hydration energy becomes 3 tim es larger than the experimental one. Thus, an over-screening may have a dra matic effect on the formation of the hydrated electron. For a reduced heigh t of the "over-screening peak", the ground state energy reduces to the meas ured value. At the same time, over-screening enhances the localization of e lectrons. The undamped resonance gives rise to an unphysically small locali zation radius. A reduced resonance, that provides the correct ground stage energy, is better in this respect but it still gives very compact localizat ion: 2/3 of the Bohr radius. It is thus concluded that either the defect st ructure of water around the electron suppresses the resonance, or the model s of bulk water, which predict a high peak in the response function, are in adequate. The study paves the way to future molecular or phenomenological m ulti-order parameter models, in which the density and polarization of molec ular dipoles and charge density distributions of the solvated electron are considered on the same footing. Such models might reveal the reduction of o ver-screening near the excess electron.