The Solvation Entropy of Different Simulation Models of the Hydrated Electron

Understanding the solvation structure of the hydrated electron, an excess electron in bulk water, has been a long-standing challenge. Experiments have shown that the solvation entropy, which encodes how the water molecules near the electron behave, is anomalously large and positive for the hydrated electron. Here, we use semiclassical and <i>ab initio</i> alchemical simulations to calculate the solvation entropy of several simulation models of the hydrated electron, including <i>ab initio</i> density functional theory (DFT) with a hybrid GGA functional. We find that cavity-forming one-electron models with relatively soft cavities correctly predict the sign of the hydrated electron's solvation entropy but underestimate its magnitude. Both a noncavity one-electron model and hard cavity-forming DFT yield an incorrect sign for the solvation entropy. The calculated solvation entropies of each model are consistent with the structure and dynamic behavior of the first-shell waters molecules.