Density Functional Theory Based Methods for the Calculation of X-ray Spectroscopy

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• X-ray spectroscopy techniques (XPS, XAS, XES, RIXS) are powerful analytical tools for studying liquids, materials, and biological systems.
• Time-resolved measurements enable the resolution of ultrafast chemical processes at an atomic level.
• Interpreting experimental X-ray spectroscopy data often relies on computational methods to understand molecular and electronic structures.

Purpose of the Study:

• To present recent contributions to simulating X-ray spectroscopic techniques using density functional theory (DFT) and linear-response time-dependent density functional theory (TDDFT).
• To demonstrate how these computational methods provide a toolkit for simulating X-ray spectroscopy and interpreting experimental results.
• To highlight the application of these methods in studying various systems, including liquids, materials, and biological molecules.

Main Methods:

• Utilizing DFT and linear-response time-dependent DFT (TDDFT) for the simulation of X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS).
• Investigating the impact of exchange-correlation functionals, including short-range corrected functionals, on XAS calculations.
• Applying efficient TDDFT implementations to study large systems and extending TDDFT to calculate XES using a reference determinant for core-ionized states.

Main Results:

• Demonstrated the importance of the exchange-correlation functional and basis set choice for accurate XAS and XES spectral simulations.
• Showcased the application of TDDFT for simulating X-ray spectra of ionic liquids, transition metal complexes, and organic molecules.
• Illustrated the ability to capture ultrafast dynamics in the femtosecond timescale from RIXS spectra simulations, such as for water.

Conclusions:

• DFT and TDDFT provide a robust computational toolkit for simulating a wide range of X-ray spectroscopic techniques.
• These methods are crucial for revealing underlying molecular structure, electronic structure, and bonding, aiding in the interpretation of experimental data.
• The accurate simulation of X-ray spectra, including going beyond the dipole approximation and careful treatment of core-excited states, is essential for understanding complex chemical phenomena and dynamics.