Abstract
The accuracy of non-resonant and resonant (resonant inelastic X-ray scattering) X-ray emission spectra simulated based upon Kohn-Sham density functional theory is assessed. Accurate non-resonant X-ray emission spectra with the correct energy scale are obtained when short-range corrected exchange-correlation functionals designed for the calculation of X-ray absorption spectroscopy are used. It is shown that this approach can be extended to simulate resonant inelastic X-ray scattering by using a reference determinant that describes a core-excited state. For this spectroscopy, it is found that a standard hybrid functional, B3LYP, gives accurate spectra that reproduce the features observed in experiment. However, the ability to correctly describe subtle changes in the spectra arising from different intermediate states is more challenging and requires averaging over conformations from a molecular dynamics simulation. Overall, it is demonstrated that accurate non-resonant and resonant X-ray emission spectra can be simulated directly from Kohn-Sham density functional theory.
Highlights
The development of advanced synchrotron sources and free-electron lasers has greatly advanced the capability of spectroscopic techniques in the X-ray region
The calculation of non-resonant and resonant (RIXS) X-ray emission spectroscopy based upon Kohn-Sham DFT has been studied
For non-resonant X-ray emission spectroscopy, short-range corrected functionals that have been designed for the prediction of X-ray absorption energies with TDDFT provide accurate spectra with the correct energy scale
Summary
The development of advanced synchrotron sources and free-electron lasers has greatly advanced the capability of spectroscopic techniques in the X-ray region. It has been shown that accurate non-resonant X-ray emission spectra can be computed within the framework of equation of motion coupled cluster theory including single and double (EOM-CCSD) excitations This is achieved by using a reference determinant that describes the core-ionised state, and the X-ray emission transitions appear as negative eigenvalues.. While this approach can provide accurate emission energies, its applicability is limited by its computational cost and the difficulty in converging a CCSD calculation for a core-hole state These problems can be overcome through the use of TDDFT; standard exchange-correlation functionals lead to an overestimation of the valence to core transition energies. The simplicity and low computational cost of obtaining X-ray emission spectra directly from a Kohn-Sham DFT calculation are very useful since it allows large systems to be studied and extensive averaging over conformation to be performed. A current challenge for computational methods is the simulation of RIXS, and we explore the extension of Kohn-Sham DFT to describe RIXS spectra
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