Modeling L_2,3-Edge X-ray Absorption Spectroscopy with Real-Time Exact Two-Component Relativistic Time-Dependent Density Functional Theory

Abstract

X-ray absorption spectroscopy is a powerful technique to probe local electronic and nuclear structure. There has been extensive theoretical work modeling K-edge spectra from first principles. However, modeling L-edge spectra directly with density functional theory poses a unique challenge requiring further study. Spin-orbit coupling must be included in the model, and a noncollinear density functional theory is required. Using the real-time exact two-component method, we are able to variationally include one-electron spin-orbit coupling terms when calculating the absorption spectrum. The abilities of different basis sets and density functionals to model spectra for both closed- and open-shell systems are investigated using SiCl and three transition metal complexes, TiCl, CrOCl, and [FeCl]. Although we are working in the real-time framework, individual molecular orbital transitions can still be recovered by projecting the density onto the ground state molecular orbital space and separating contributions to the time evolving dipole moment.

Fingerprinting Electronic Structure of Heme Iron by Ab Initio Modeling of Metal L-Edge X-ray Absorption Spectra

“…A number of models without system-specific parameters have also been used to describe metal L-edge XAS spectra of molecular systems. [28][29][30][31][32][33][34] Multiconfigurational wavefunction approaches are well suited to treat correlation in transition metal systems, among which the complete active space is most widely used. X-ray processes typically involve only single core-orbital excitations, which makes it convenient to use a restricted active space (RAS) wavefunction.…”

Efficient and low-scaling linear-response time-dependent density functional theory implementation for core-level spectroscopy of large and periodic systems

“…Since XAS LR-TDDFT spectra need to be rigidly shifted anyways, scalar relativistic effects can usually be assumed to be accounted for that way. 11 Adding SOC to TDDFT is less trivial and generally involves more computationally intensive methods such as two- 56,57 or even four-component 58 relativistic TDDFT. A simpler approach consists of adding SOC as a perturbation to a non-relativistic calculation by coupling excited state wave functions of different spin multiplicities.…”

“…For nitrous oxide, N T represents the terminal nitrogen and N C the central nitrogen. The standard GGA functionals BLYP 50,51 and PBE 52 as well as hybrid functionals with increasing fraction of Hartree-Fock exchange B3LYP (20%), 53 PBE0 (25%) 54 and BHHLYP (50%) 55 56 who use a more advance two-component relativistic TDDFT scheme. Despite the comparative simplicity of our method, the spectra we obtain are very similar.…”

Relativistic four-component linear damped response TDDFT for electronic absorption and circular dichroism calculations

“…However, the methodology suffers from an increased computational cost for many chemical applications where it suffices to describe molecular response properties to the first few lowest orders for a narrow frequency range. Implementations have been reported at the 4c level of theory and used to calculate electron absorption spectra in valence 52 and X-ray 53 regions and circular dichroism spectra 54 , as well as at the 2c X2C level of theory and applied to electron absorption spectra in the valence 55 and X-ray 56 regions, and non-linear optical properties 57 . This work focuses on the linear damped response theory, the derivation of its work-ing equation based on time-dependent perturbation theory formulated for general multicomponent spinor cases (2c, 4c), and the description of a multi-frequency iterative subspace solver accounting for hermicity and time-reversal symmetry.…”