[DFTB-Plus-User] R: Different Fermi energy in DFTB+ in compare to QE

[Alessandro Pecchia]

Dear Mohammad,

 

the Fermi Energy of a periodic supercell is not particularly meaningful and does not directly compares to the work function. Different codes can lead to very different numbers and this is normal. 

 

You may expect that for a GNR QE should reproduce an accurate workfunction. In any case a lot depends on the supercell size along the dummy directions. The workfunction is obtained by looking at the potential profile and checking its value in the vacuum region, in between periodic replicas. Then you need to take the difference between the vacuum potential and the computed Fermi energy. Tutorials about this can be found on QE manuals and tutorial examples.

 

Concerning dftb+, the procedure would be the same, but the potential between replicas will probably differ from QE and for your system will be probably close to 0. 

It should be said that in general dftb+ does not reproduce very accurate workfunctions.

 

I hope this helps,

 

 

Alex Pecchia

 

Da: DFTB-Plus-User [mailto:dftb-plus-user-bounces at mailman.zfn.uni-bremen.de] Per conto di Mohammad Amir Bazrafshan
Inviato: sabato 11 giugno 2022 00:30
A: dftb-plus-user at mailman.zfn.uni-bremen.de
Oggetto: [DFTB-Plus-User] Different Fermi energy in DFTB+ in compare to QE

 

Dear all

 

I am confused because the calculated Fermi energy in DFTB+ and QE are different.

I have done band structure calculations for 7-AGNR (with identical input geometry). 

QE says: 
the Fermi energy is    -2.6171 ev 
, and DFTB+ says : 
Fermi level:                        -0.1901870389 H           -5.1753 eV.

Is there any explanation for this? Or my calculations have some problems? I should mention that, in general, the band structures are similar to each other.

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