## Binding energy of C 1s by ICORELEVEL tag

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### Binding energy of C 1s by ICORELEVEL tag

Dear friends,
I came across a problem when I used ICORELEVEL=2 to calculate C 1s binding energy in diamond.
The total energy of groundstate TOTEN of an 8-atom-thick diamond-(001) slab is E(1)=-132.09 eV. The TOTEN of the system with one C 1s electron excited to the conduction band is E(2)= -188.06 eV. According to the pioneering work (Density functional study of CO on Rh(111), Phys. Rev. B, 2004, 165405), the binding energy of core electron is calculated by ECL=E(nc‒1)-E(nc). In the case of diamond-(001), however, the binding energy of C1s thus should be ECL=E(2)-E(1)= -55.97 eV. The standard experimental value is 285.6 eV.
I don’t know how to explain the deviation between the calculated and experimental results.

The POSCAR used in the two runs (has been fully relaxed before the 2 runs):
Diamond_001
1.00000000000000
3.5324834968469405 0.0000000000000000 0.0000000000000000
0.0000000000000000 3.5324834968469405 0.0000000000000000
0.0000000000000000 0.0000000000000000 23.5918006897000012
C
16
Direct
0.5000000000000000 -0.0000000000000000 0.4347635844316595
0.5000000000000000 0.0000000000000000 0.5835807119621851
0.0000000000000000 -0.0000000000000000 0.3578566809512059
-0.0000000000000000 0.0000000000000000 0.5105247687964733
0.5000000000000000 0.5000000000000000 0.3578566809512059
0.5000000000000000 0.5000000000000000 0.5105247687964733
0.0000000000000000 0.5000000000000000 0.4347635844316595
0.7500000000000000 0.7500000000000000 0.3973452552035243
0.7500000000000000 0.7500000000000000 0.5500133130487961
0.2500000000000000 0.7500000000000000 0.3242893120378195
0.2500000000000000 0.7500000000000000 0.4731064095683427
0.2500000000000000 0.2500000000000000 0.3973452552035243
0.2500000000000000 0.2500000000000000 0.5500133130487961
0.7500000000000000 0.2500000000000000 0.3242893120378195
0.7500000000000000 0.2500000000000000 0.4731064095683427
-0.0000000000000000 0.5000000000000000 0.5835807119621851

The INCAR in the second run:
SYSTEM=diamond_001
ENCUT=500
ISTART=0; ICHARG=2
EDIFF=1E-6
NELM=200
EDIFFG=-0.005

ISIF=3
IBRION=-2; NSW=0
LREAL=Auto
NCORE=4

ICORELEVEL=2
CLNT=2
CLN=1
CLL=0
CLZ=1