Efficient simulation of near-edge x-ray absorption fine structure (NEXAFS) in density-functional theory: Comparison of core-level constraining approaches

Abstract

Widely employed Near-Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy probes a system by excitation of core electrons to unoccupied states. A variety of different methodologies are available to simulate corresponding spectra from first-principles. Core-level occupation constraints within ground-state Density-Functional Theory (DFT) represent a numerically most efficient means to this end that provides access to large systems, examples being surface adsorption, proteins, polymers, liquids, and buried, condensed phase interfaces (e.q. solid-liquid and solid-solid). Here, we systematically investigate the performance of different realizations of this approximate approach through the simulation of K-edge NEXAFS-spectra of a set of carbon and nitrogen-containing organic molecules. Variational collapse to the ground state and oscillatory convergence are the major complications of these approximate computational protocols. We present a modified version of the maximum-overlap method to achieve a self-consistent inclusion of electrons in virtual states for systems where convergence is hampered due to degeneracies. Our results demonstrate that reliable spectra allowing for a semi-quantitative analysis of experimental data are already obtained at the semi-local level of density functionals and with standard numeric atomic orbital basis sets.

Relationship between Hydrogen-Bonding Motifs and the 1b1 Splitting in the X-ray Emission Spectrum of Liquid Water

“…This likely arises from the well-known Delta SCF limitation in describing the excited state as not orthogonal to the variational ground state of the system. 45…”

“…57,58 However, due to non-orthogonality between the variational ground state and an excited state obtained through Delta SCF, Delta SCF calculations suffer from inherent limitations, often leading to incorrect localization of ionized states which negatively impact their accuracy. 45 Specific details about the present QM/MM TD-DFT calculations are reported in the Supporting Information.…”

Accurate Absolute and Relative Core-Level Binding Energies from GW

“…Constraining the core hole in a particular state can be difficult, and variational instabilities are not uncommon. 19 …”

XABOOM: An X-Ray Absorption Benchmark of Organic Molecules Based on Carbon, Nitrogen, and Oxygen 1s → π ∗ Transitions

“…Of note is the work of Besley and Asmuruf, 33 who investigated the performance of TDDFT for core properties and the construction of functionals with reasonable absolute energies, as well as numerous smaller studies for different methods. [34][35][36][37][38][39][40][41][42][43][44] In order to accurately model core-excitation processes, the inclusion of electron relaxation effects is vital. This requires a theoretical method capable of capturing two physical effects: a reduced screening of the probed nuclei following the removal of a core electron, leading to a strong net attraction of the electron density towards the core, and a smaller repulsive polarisation effect in the valence region due to the interaction with the excited electron.…”

“…An abundance of methodologies for modeling X-ray absorption spectra has been developed, including semi-empirical, density-based, and wave function-based methods. 31,33,34,[36][37][38][39]43,45,[49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68] Here the focus is on first-principles methods, and hence semi-empirical approaches will thus be left out of the discussion. Among the first-principles methods, researchers in the field of theoretical spectroscopy commonly apply density functional theory (DFT).…”