Simulating X-ray Absorption and Emission Spectra with TDDFT

X-ray absorption (XAS) and emission (XES) spectroscopy

  • Difficulty: core-excited states are too high in energy to access through standard bottom-up algorithms for computing excited states.

  • Solution: set up a reduced excitation space that includes only relevant core orbitals in the occupied space (as in the core-valence separation scheme).

  • Valence-to-core XES transitions are obtained through a TDDFT calculation with a non-Aufbau reference state containing a core hole.

Advantages of using TDDFT for simulating X-ray spectroscopy:

  • Good balance between computational accuracy and cost. Can be applied to realistic molecular systems;

  • With special-purpose xc functionals (e.g., SRC functionals in Q-Chem), errors in core excitation energies are significantly reduced;

  • Only minimal input parameters are necessary (almost).

Simulated XAS for butadiene

Simulated XES for acetone

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