Quantum simulations of electron dynamics in aqueous systems

俄亥俄州立大学的John M. Herbert获得化学学部化学理论、模型和计算方法（CTMC）项目的奖励，研究溶剂化电子的性质和反应性及其对生物系统的影响。高能辐射与生物体中的水的相互作用会产生一系列高活性物质，这些物质可以继续破坏生物材料。在这些活性物质中有所谓的“水合”或“溶剂化”电子。溶剂化电子在50多年前首次被发现，但详细的结构信息仍然缺乏。这部分是由于其超短的寿命，通常不超过几微秒。这给实验带来了困难，赫伯特正在进行理论和计算研究，以模拟溶剂化电子的结构和性质，以及它与DNA关键分子组分的相互作用。这些研究接触了一种有前途的新实验技术，该技术可以直接探测液体系统的能级结构，但通常需要理论计算来解释其结果。该项目的最终目标是了解这些溶剂化电子是否能够直接破坏DNA。为了实现这一目标，人们正在开发新的理论和计算工具。其中包括用于从头算分子动力学模拟的新模拟协议，采用连续体边界条件，可以与基于高斯轨道（而不是基于平面波）的密度泛函理论（DFT）代码结合使用。这些发展允许使用精确的混合函数，这对于平面波模拟来说太昂贵了。此外，由于高能辐射的沉积可以产生激发态的分子，改进版本的时间依赖（TD-）DFT正在开发中，它可以在保持适当的自旋（S2）对称性的同时适当地描述锥形交叉。这些方法与“实时”TD-DFT一起被用于模拟水系统中的电子注入和去除（光电发射）。

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.