Modeling X-ray Transient Spectroscopy with Adaptive Wavefunction Methods

Emory University, Atlanta GA

Investigators

Abstract

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 states This 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.