Quantum simulations of electron dynamics in aqueous systems

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John M. Herbert at The Ohio State University is supported by an award from the Chemical Theory, Models and Computational Methods (CTMC) Program in the Chemistry Division to study the properties and reactivity of solvated electrons and their impacts on biological systems. The interaction of high-energy radiation with water in organisms generates a cascade of highly reactive species that can go on to damage biological material. Amongst these reactive species is the so-called "hydrated" or "solvated" electron. The solvated electron was first discovered more than 50 years ago but detailed structural information is still lacking. This is due in part to its ultra-short lifetime, generally not more than a few microseconds. This poses experimental difficulties, and Herbert is conducting theoretical and computational studies to simulate the structure and properties of the solvated electron and its interaction with the key molecular components of DNA. These studies make contact with a promising new experimental technique which can provide a direct probe of the energy-level structure of liquid systems, but for which theoretical calculations are often required to interpret its results. The ultimate goal of the project is to understand whether these solvated electrons are capable of directly damaging DNA. In order to accomplish this goal, new theoretical and computational tools are being developed. These include new simulation protocols for ab initio molecular dynamics simulations, employing continuum boundary conditions that can be used in conjunction with a Gaussian-orbital-based (rather than plane-wave-based) density functional theory (DFT) code. These developments allow use of accurate hybrid functionals, which are too expensive for plane-wave simulations. In addition, because the deposition of high-energy radiation can generate molecules in excited states, improved versions of time-dependent (TD-)DFT are being developed that can properly describe conical intersections while retaining proper spin (S2) symmetry. These methods are being used, along with "real-time" TD-DFT, to simulate electron injection and removal (photoemission) in aqueous systems.