New Coupled-cluster Methods for Modeling Valence and Core-level Spectroscopy in Bound and Unbound Domains

Anna Krylov of the University of Southern California is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to extend the theoretical spectroscopy modeling toolkit by developing novel electronic structure methods in the core and valence domains with particular emphasis on free-electron states. Using light to interrogate matter is the basis of spectroscopy, which provides the most powerful set of tools for unraveling mechanisms of chemical reactions, structures and intrinsic properties of materials and biological objects. Using high-energy (X-rays) and high-intensity radiation opens new exciting opportunities, which motivate the worldwide development of multi-billion-dollar facilities for advanced light sources. Recent advances in beam quality in these facilities greatly expanded possible applications of X-rays, giving rise to a proliferation of techniques including those operating in time-resolved and non-linear regimes. These novel techniques promise to greatly expand our ability to interrogate molecular structure and dynamics, but their full potential can only be realized when experiment is augmented by accurate theoretical tools for modeling these phenomena. Despite significant efforts, the theory is still lagging behind the experimental capabilities, creating a bottleneck for maximizing the scientific impact of multi-billion advanced light source facilities. One of the challenges is that many techniques involve states in the continuum—autoionizing resonances as well as photoejected, Auger, or scattered electrons— which are not amenable to standard quantum-chemistry techniques. This proposal aims to bridge this gap by developing novel electronic structure methods for modeling spectroscopy in the core and valence domains with particular emphasis on free-electron states. Broader impact of the proposed research includes training and mentoring of graduate students and postdocs for careers in academia and industry as well as contributions to research infrastructure by integrating new computer codes in the widely used ab initio programs Q-CHEM and SPARTAN to make them available to the broad chemistry community. Krylov will (i) develop a novel ab initio framework for treating free-electron states using plane-wave-Gaussian basis sets; (ii) use this new framework to implement calculations of photo- electron circular dichroism (PECD), angular-resolved photoelectron cross sections, and extended x-ray absorption fine structure (EXAFS) and to improve the theoretical treatment of Auger and related phenomena such as intermolecular Coulomb decay (ICD) and electron-transfer mediated decay (ETMD); (iii) include spin–orbit effects in calculating Auger spectra for L and M edges; (iv) extend theory to the modeling of spectroscopy with magnetic fields such as magnetic CD (MCD and X-MCD); (v) extend Cholesky decomposition approach to calculations of core-level states within equation-of-motion coupled-cluster theory. 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.