Axial ligand-related issues in XAFS calculations

[Eason Jane]

Dear Community，

  I would like to ask a question regarding a recent calculation. After adding an O2 molecule as an axial ligand to an Fe-N4 structure, the calculated XAFS spectrum using CP2K shows an intense peak at ~6999 eV. Could anyone kindly provide some insights into why this happens? I have found that this specific feature appears whenever the Fe center is bonded to O or S atoms. 

  （ GLOBAL ： RUN_TYPE ENERGY    theoretical method：PBEh_ADMM with HFX of 45.000 % without RI-HFX    basis set and pseudopotential：DZVP-MOLOPT-SR-GTH     OT   GAPW     TD-DFT

   Fex ： POTENTIAL ALL  BASIS_SET：def2-TZVP）

  Thank you in advance for your time and help.

Eason