Modeling X-ray Transient Spectroscopy with Adaptive Wavefunction Methods

埃默里大学的Francesco Evangelista得到了化学系化学理论、模型和计算方法项目的一个奖项的支持，以开发计算方法来研究 分子如何相互作用并在暴露于X射线辐射时发生变化。X射线可用于探测化学反应过程中电子和原子核的动力学。 这些实验 一个前所未有的时间分辨率下降到阿秒尺度（一万亿分之一秒）。通过X射线探测分子的实验的解释在很大程度上依赖于量子力学模拟。 这涉及到解决这些系统的电子薛定谔方程。 由于相对论效应和电子的相关运动的结合，这样的计算是具有挑战性的。Evangelista和他的同事们为这种模拟开发了有效而准确的方法。 他们开发了有效的自适应算法，降低了模拟分子中电子结构和动力学的计算成本。他们还开发 开源软件，实现了本项目中开发的理论。 该软件可供更广泛的研究界使用。 Evangelista还招募了STEM专业的本科生，在他的研究小组中进行暑期研究。 在基本层面上，该项目解决了计算瞬态核心激励的问题，弱相关性和强相关性都有重要贡献。Evangelista采用的策略使用紧凑的选择组态相互作用波函数来描述强相关效应和有效的多体相关方法来处理弱相关。本计画所发展的自适应组态相互作用（ACI）方案具有自适应性与可调性，并提供一个系统的路径来计算高精度的激发态。ACI方法的开发是为了解决两个具有挑战性的问题，需要明确的治疗高度激发的电子配置：瞬态X射线光谱的模拟和核心电离和核心激发态的衰变。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Francesco Evangelista of Emory University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to develop computational approaches to study how molecules interact and change in response to exposure to X-ray radiation. X-rays can be used to probe the dynamics of electrons and nuclei during chemical reactions. These experiments have an unprecedented time resolution down to the attosecond scale (one quintillionth of a second). The interpretation of experiments that probe molecules via X-rays relies heavily on quantum mechanical simulations. This involves solving the electronic Schrodinger equation for these systems. Such calculations are challenging due to the combination of relativistic effects and the correlated motion of electrons. Evangelista and his coworkers develop efficient and accurate methods for such simulations. They develop efficient adaptive algorithms that reduce the computational cost of simulating the structure and dynamics of electrons in molecules. They also develop open-source software that implements the theories developed in this project. This software is made available to the broader research community. Evangelista also recruits undergraduates majoring in STEM fields to pursue summer research in his research group. At a fundamental level, the project addresses the problem of computing transient core-excitations with important contributions from both weak and strong correlation. The strategy employed by Evangelista uses compact selected configuration interaction wave functions to describe strong correlation effects and efficient many-body correlation methods to treat weak correlations. The adaptive configuration interaction (ACI) scheme developed in this project is both adaptive and tunable and provides a systematic path to compute excited states with high accuracy. The ACI method is developed to address two challenging problems that require an explicit treatment of highly-excited electronic configurations: the simulation of transient X-ray spectroscopies and the decay of core-ionized and core-excited statesThis award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.