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

俄亥俄州立大学的John Herbert获得了化学理论、模型和计算方法项目的奖励，以开发改进的基于电子结构的方法，用于模拟凝聚态系统中的电子激发和电离，特别是大分子和水溶液。赫伯特和他的研究小组开发了基于自旋翻转时间依赖密度泛函数理论的算法，以定位沿锥形接缝的最小能量交叉点，这使他们能够探索和表征大型系统中的激发态势能表面，例如具有多个核碱基的完全溶剂化DNA，量子力学描述。一个主要的目标是在DNA中进行飞秒激光光谱测量。量子力学/分子力学（QM/MM）方法也正在为此目的而开发，这些方法与QM区域的任意描述相兼容。QM/MM方法对溶剂电离后的极化响应进行了不同的描述，正在开发和测试用于计算液体溶液中分子的垂直电离能；这些量现在可以通过液体微射流光电子能谱来测量。改进的、数值鲁棒的连续介质静电学版本正在开发中，作为这些模拟的低成本边界条件。时间分辨（飞秒）激光光谱学是研究复杂系统分子水平化学动力学的重要工具，但这些实验通常需要详细的量子力学计算和分子动力学模拟的帮助，以便对实验测量的时间常数和/或能量间隙进行机械解释。赫伯特研究小组正在开发计算工具，以在分子水平上模拟这些实验。虽然他们开发的计算机器和软件相当通用，但赫伯特和他的同事们专注于几个重要的化学问题。其中包括紫外线激发DNA后的光物理学，他们试图了解输入能量的去向，它是如何破坏分子的，以及存在什么样的“陷阱”来防止这种破坏。他们也对液态水和其他水系统的电离感兴趣，以此来理解水辐射分解过程的动力学和能量学。

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.