The calculated XAFS spectrum for the Fe-N4 structure with an added axial ligand shows an intense peak at ~6999 eV

[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. 

  Thank you in advance for your time and help.

Eason