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、模型和计算方法计划的支持，该计划开发计算方法来研究分子如何相互作用和变化，以响应X射线辐射。X射线可以用来探测化学反应中电子和原子核的动力学。这些实验具有史无前例的时间分辨率，精确到阿秒(五分之一秒)。对通过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。这涉及到求解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。伊万杰利斯塔和他的同事们为这种模拟开发了高效而准确的方法。他们开发了高效的自适应算法，降低了模拟分子中电子的结构和动力学的计算成本。他们还开发实现该项目中开发的理论的开源软件。该软件可供更广泛的研究社区使用。Evangelista还招募STEM领域的本科生在他的研究小组进行暑期研究。在基本层面上，该项目解决了利用弱关联和强关联的重要贡献来计算瞬变核激发的问题。Evangelista所采用的策略使用紧凑的选定组态相互作用波函数来描述强关联效应，并使用有效的多体关联方法来处理弱关联。本项目中开发的自适应组态相互作用(ACI)方案既是自适应的，又是可调的，为高精度计算激发态提供了系统的途径。ACI方法是为了解决两个具有挑战性的问题而开发的，这两个问题需要明确处理高激发电子组态：瞬时X射线光谱的模拟以及核电离和核激发态的衰变。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.