Evaluating the\nChemical Reactivity of DFT-Simulated\nLiquid Water with Hydrated Electrons via the Dual Descriptor

Modeling the various properties of liquid water, particularly\nits\nreactivity, has been a longstanding challenge for simulation methods.\nRecently, <i>ab initio</i> simulations based on density\nfunctional theory (DFT) have come to the fore as tenable methods for\ncalculating the properties and reactivity of water, with varying degrees\nof success for different exchange-correlation functionals. In particular,\nhybrid-GGA and meta-GGA functionals have been shown to reproduce many\nof the structural, dynamical, and energetic properties of water to\na high degree of accuracy relative to their computational cost. Here,\nwe show that the dual descriptor (DD) measure of nucleophilicity and\nelectrophilicity, which is sometimes used to elucidate organic chemistry\nreaction mechanisms, can also be used to characterize the reactivity\nof DFT-simulated liquid water. The DD is especially apt for understanding\nthe reactivity of excess electrons with water as its calculation explicitly\ninvolves adding and removing an excess electron from a reference system.\nWe use the DD to explore the reactivity of water simulated using three\ndifferent DFT functionals: the LDA functional (LDA), a hybrid-GGA\nfunctional (PBE0), and a hybrid meta-GGA functional (SCAN0). Using\nthe DD, we show that the SCAN0 functional with the standard 25% Hartree–Fock\nexchange produces simulated liquid water with many regions that are\nfar more reactive than either PBE0 or LDA. To understand the implications\nof these highly reactive regions, we then add a strong nucleophile\nin the form of an excess electron and find that although PBE0 and\nLDA predict stable hydrated electrons, the excess electron reacts\nnearly instantaneously with SCAN0 water via proton abstraction to\nform a hydrogen atom and hydroxide ion. We show that the DD provides\nthe ability to not only predict whether or not liquid water will react\nwith a hydrated electron but also which particular waters will be\ninvolved solely from analyzing pure water configurations generated\nwith each functional. We rationalize this result in terms of the known\ntrap-seeking behavior of injected hydrated electrons, which are able\nto find the most electronegative region in bulk water. These results\nhighlight the utility of the dual descriptor as a fast and interpretable\nmethod for investigating condensed-phase reactivity with excess electrons.