A time-dependent density functional theory protocol for resonant inelastic X-ray scattering calculations

TL;DR

This paper introduces a TDDFT-based protocol for calculating resonant inelastic X-ray scattering spectra, enabling accurate simulations of complex molecular systems with computational efficiency.

Contribution

The authors develop a novel TDDFT approach using pseudo-wavefunctions and a restricted energy window to compute RIXS spectra, improving accuracy and efficiency.

Findings

Accurately reproduces experimental RIXS maps for Ruthenium complexes

Captures spectral features with energy errors within 0.6 eV

Requires only two TDDFT calculations per system

Abstract

We present a time-dependent density functional theory (TDDFT) based approach to compute the light-matter couplings between two different manifolds of excited states relative to a common ground state. These quantities are the necessary ingredients to solve the Kramers--Heisenberg equation for resonant inelastic X-ray scattering (RIXS) and several other types of two-photon spectroscopies. The procedure is based on the pseudo-wavefunction approach, where TDDFT eigenstates are treated as a configuration interaction wavefunction with single excitations, and on the restricted energy window approach, where a manifold of excited states can be rigorously defined based on the energies of the occupied molecular orbitals involved in the excitation process. We illustrate the applicability of the method by calculating the 2p4d RIXS maps of three representative Ruthenium complexes and comparing them to experimental results. The method is able to accurately capture all the experimental features in all three complexes, with relative energies correct to within 0.6 eV at the cost of two independent TDDFT calculations.