On the Performance of Optimally Tuned Range-Separated Hybrid Functionals for X-ray Absorption Modeling.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Accurate modeling of X-ray absorption spectra (XAS) is crucial for understanding molecular electronic structure.
• Traditional density functional theory (DFT) methods often struggle to precisely reproduce experimental XAS features.
• Range-separated hybrid (RSH) functionals offer improved electronic structure descriptions but require careful parameterization.

Purpose of the Study:

• To evaluate the performance of optimally tuned range-separated hybrid (OT-RSH) functionals for simulating XAS.
• To compare OT-RSH performance against empirical functionals for a benchmark set of simple molecules.
• To develop a correction scheme for calculating XAS on an absolute energy scale.

Main Methods:

• Time-dependent density functional theory (TDDFT) combined with the Path Integral based Reflection Principle methodology.
• Simulation of X-ray absorption spectra for water, ammonia, methane, hydrogen peroxide, hydrazine, and ethane.
• Optimization of range separation parameters for each molecular geometry and comparison with experimental data.

Main Results:

• OT-RSH functionals demonstrate superior accuracy in predicting relative peak positions and intensities compared to empirical functionals.
• Geometry-specific tuning of the range separation parameter further enhances spectral prediction accuracy.
• A novel correction scheme enables absolute energy scale calculations for XAS using OT-RSH and ΔSCF/TDDFT.

Conclusions:

• Optimally tuned range-separated hybrid functionals represent a significant advancement for accurate XAS modeling.
• The proposed correction scheme provides a pathway for precise absolute energy predictions in XAS.
• This work establishes a robust computational framework for studying molecular electronic properties via XAS.