New methods for describing electronic excitation, ionization, and electrostatics of complex systems in aqueous environments

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John Herbert of the Ohio State University is supported by an award from the Chemical Theory, Models and Computational Methods program to develop improved electronic structure-based methods for simulating electronic excitation and ionization in condensed-phase systems, specifically macromolecules and aqueous solutions. Herbert and his research group develop algorithms to locate minimum-energy crossing points along conical seams, based on spin-flip time-dependent density functional theory, which allows them to explore and characterize excited-state potential energy surfaces in large systems such as fully-solvated DNA with multiple nucleobases described quantum-mechanically. A major goal is to make contact with femtosecond laser spectroscopic measurements in DNA. Quantum Mechanics/Molecular Mechanics (QM/MM) methods that are compatible with an arbitrary description of the QM region are being developed for this purpose as well. QM/MM methods, with various descriptions of the solvent's polarization response following ionization, are being developed and tested for calculation of vertical ionization energies of molecules in liquid solution; such quantities can now be measured via liquid microjet photoelectron spectroscopy. Improved, numerically-robust versions of continuum electrostatics are being developed as low-cost boundary conditions for these simulations.Time-resolved (femtosecond) laser spectroscopy is an important tool for interrogating the molecular-level chemical dynamics of complex systems, but often these experiments need assistance from detailed quantum-mechanical calculations and molecular dynamics simulations in order to assign a mechanistic interpretation to the time constants and/or energy gaps that are measured experimentally. The Herbert research group is developing the computational tools to simulate these experiments at the molecular level. Although the computational machinery and software that they develop is quite general, Herbert and his coworkers focus on a few important chemical problems. These include the photophysics following UV excitation of DNA, where they seek to understand where the input energy goes, how can it damage the molecule, and what sorts of "traps" exist to prevent such damage. They are also interested in the ionization of liquid water and other aqueous systemsas a means of understanding the dynamics and energetics of the water radiolysis process.