Q-Chem Webinar 74: Electron-Affinity TDDFT: How Better Starting Points Beget Better Results in TDDFT Calculations of X-ray Absorption Spectra (Kevin Carter-Fenk)

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Advancements in synchrotron and tabletop light sources have made the study of core-level photochemistry in the x-ray regime more accessible, providing unprecedented insights into the interplay of electronic and nuclear structure. While experimental technology bounds ahead, common computational methods such as time-dependent density functional theory (TDDFT) face significant challenges, particularly with x-ray absorption spectra (XAS), where errors can exceed 10 eV. Such substantial errors hinder our ability to confidently offer theoretical insights into real-world phenomena.

In this webinar, we will explore the idea that the optimal orbitals for the ground state are not simultaneously optimal for core-excited states. By shifting our reference density in TDDFT calculations from the ground-state to core-ionized states, we better align our starting point with the core-excited states that we aim to model, thus accounting for orbital relaxation a priori. Then, through careful elimination of self-interaction errors in the TDDFT response, we arrive at electron-affinity TDDFT (EA-TDDFT), a method achieving errors as low as 0.5 eV for K-edge excitation energies at the cost of TDDFT.

We investigate the near-edge x-ray absorption fine structure (NEXAFS) spectroscopy of various aqueous solutions through the lens of EA-TDDFT, showing that while TDDFT requires a trade-off between accurate energies and intensities, EA-TDDFT gives excellent agreement with experimental data for both quantities. After establishing that EA-TDDFT provides accurate liquid-phase spectra, we extend its application to the NEXAFS of glycerol aerosols. Combining molecular dynamics simulations with EA-TDDFT, we reveal the significance of quantum nuclear effects in modeling the NEXAFS spectrum of glycerol at and below 300 K. Finally, we propose that glycerol aerosols spontaneously form glasses upon aerosolization, resulting in strong H-bond interactions that broaden the aerosol-phase NEXAFS features due to a phenomenon that we term as a “purple-shift”. Our results establish EA-TDDFT as a powerful and inexpensive tool in the quantum chemist’s toolkit for computing XAS in both gas and condensed-phase systems, opening the door to exciting new insights into the interplay nuclear and electronic structure.

About the presenter:
Kevin Carter-Fenk earned B.S. and Ph.D. degrees in Chemistry from The Ohio State University, where they worked under Prof. John Herbert on a dissertation entitled “Design and Implementation of Quantum Chemistry Methods for the Condensed Phase: Noncovalent Interactions at the Nanoscale and Excited States in Bulk Solution”. Highlights of their dissertation work include a non-electrostatic model of pi-stacking interactions, the state-targeted energy projection (STEP) algorithm for self-consistent optimization of excited state electron configurations, and density functional theory simulations of hydrated electron formation mechanisms in aqueous solution. Kevin then worked as a Ruth L. Kirschstein Postdoctoral Fellow in Prof. Martin Head-Gordon’s group at University of California, Berkeley on time-dependent density functional theory methods for core-level spectroscopy and size-consistent Brillouin-Wigner perturbation theories. Kevin will begin an independent career at the University of Pittsburgh at the rank of assistant professor on August 1, 2024, where they will investigate the photochemistry of transition metal complexes with an eye towards sustainable energy applications.