Modeling X-ray Transient Spectroscopies with Advanced Multireference Methods

With support from the Chemical Theory, Models and Computational Methods (CTMC) program in the Division of Chemistry, Francesco Evangelista of Emory University is developing computational approaches to simulate highly reactive molecules interacting with X-ray radiation. New experimental techniques use X-rays to track the dynamics of electrons and nuclei during chemical reactions at extremely short time scales. These experiments can also reveal how molecules interact with their environment. It is difficult to extract the atomistic mechanism of reactions from X-ray experiments alone. Quantum mechanical calculations play an important role in interpreting X-ray experiments. Scientists cannot accurately perform these calculations for many interesting molecules due to the challenges created by the correlated motion of electrons. Evangelista and his research group are developing methods that will enable simulations of state-of-the-art experiments that can be used to track reactions in complex chemical environments. This project will create new open-source computer codes that implement the theories developed by Evangelista and that are freely available. The Evangelista group also plans to engage undergraduates in summer research as part of the team to help build the future STEM (science, technology, engineering and mathematics) workforce. Under this award, Francesco Evangelista and his team will develop methods to compute core-excited states for molecules in regimes where many-body correlations contribute significantly to the wave function. The approach will employ the driven similarity renormalization group (DSRG) formalism to account for dynamical correlation effects to produce an effective Hamiltonian for a manifold of electronic states. The project seeks to develop a new technique to target specific valence- and core-excited diabatic states and will apply these methods to simulate state-of-the-art experiments that track bond-dissociation processes. The group also seeks to extend their computations to molecules adsorbed on semiconductors and solid-state defect qubits by combining the GAS (general activation space)-DSRG approach with multi-reference quantum embedding methods. These techniques are intended to address the problem of computing full X-ray absorption spectra for transient species as their molecular geometry and electronic state evolve over time. The proposed methods will be general in that they will apply to both ground and valence-excited starting states. More broadly, these developments are expected contribute to the advancement of multi-reference theories with potential impact on the related areas of photochemistry and chemical reactivity. These methods are potentially deployable for the simulation of diamond defects with relevance to quantum information science (QIS) as, for example, in the use of X-ray spectroscopy for the characterization of solid-state qubits. 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.