The hydrated electron is of interest to both theorists and experimentalists as a paradigm solution-phase quantum system. Although recent work has focused on ab initio molecular dynamics to study this system, even with relatively inexpensive quantum chemistry methods such as density functional theory (DFT), such calculations are still limited to at most a few tens of water molecules and only a few picoseconds duration.  This leaves open the question as to whether the calculations are converged with respect to either system size or dynamical fluctuations. In this work, we attempt to remedy this situation by running DFT- based ab initio simulations of the hydrated electron as a function of box size, temperature, and the presence if inert ions.  We show that the calculated properties of the hydrated electron are not converged even with simulation sizes up to 128 water molecules and durations of several tens of picoseconds.  Our temperature-dependent simulations predict a red-shift of the absorption spectrum (computed using TD-DFT with an optimally tuned range-separated hybrid functional) with increasing temperature, but the magnitude of the predicted red-shift is larger than that observed experimentally, and the absolute position of the calculated spectra are off by roughly half an eV. The spectral red-shift at high temperatures is accompanied by both a partial loss of structure of the electron’s central cavity and an increased radius of gyration that pushes electron density onto and beyond the first solvation shell.  Finally, we show that the DFT hydrated electron has such a tight hydration structure that it pairs far too strongly with sodium cations, leading to a precited spectral shift that goes the opposite direction from experiment.  Overall, although ab initio simulations can provide some insights into the temperature- and ion-dependent behavior of the hydrated electron, the simulation sizes and level of quantum chemistry theory that are currently accessible are inadequate for correctly describing the experimental properties of this fascinating object.

Can DFT-based MD simulations explain the structure and properties of the hydrated electron?

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