A Combined Wave Function and Density Functional Approach for K-Edge X-ray Absorption Near-Edge Spectroscopy: A Case Study of Hydrated First-Row Transition Metal Ions

The prediction of X-ray absorption spectra (XAS) of transitionmetal complexes has important and broad application areas in chemistryand biology. In this letter, we have investigated the predictive abilityof multiconfiguration pair-density functional theory (MC-PDFT) forX-ray absorption spectra by calculating the metal K pre-edge featuresof aquated 3d transition metal ions in common oxidationstates. MC-PDFT results were compared with experimentally measuredspectra as well as analyzed against results from restricted active-spacesecond-order perturbation theory (RASPT2) and time-dependent densityfunctional theory (TDDFT). As expected, TDDFT performs well for excitedstates that can be accurately represented by singly excited configurationsbut fails for excited states where higher order excitations becomeimportant. On the other hand, both RASPT2 and MC-PDFT provide quantitativelyaccurate results for all excited states irrespective of their character.While core-level spectroscopy with RASPT2 is accurate, it is computationallyexpensive. Our results show that MC-PDFT performs equally well withsignificantly lower computational cost and is an encouraging alternateapproach for X-ray spectroscopies.

Automated summary

This letter investigates the predictive ability of multiconfiguration pair-density functional theory (MC-PDFT) for X-ray absorption spectra (XAS) by calculating the metal K pre‑edge features of aquated 3d transition metal ions in common oxidation states. It is found that MC-PDFT provides quantitatively accurate results for all excited states with significantly lower computational cost compared to restricted active-space second-order perturbation theory (RASPT2) and time-dependent density functional theory (TDDFT).