fermi energy in band structure calculation

Question on input files/tags, interpreting output, etc.

Please check whether the answer to your question is given in the VASP online manual or has been discussed in this forum previously!

Moderators: Global Moderator, Moderator

Post Reply
Message
Author
decai
Newbie
Newbie
Posts: 8
Joined: Tue May 16, 2006 4:11 pm
License Nr.: 253

fermi energy in band structure calculation

#1 Post by decai » Mon Dec 31, 2007 2:07 am

I have a question on determining the fermi energy in band structure calculation.
First, I do a self-consistent run using Tetrahedron method (ISMEAR=-5).
Second, I calculate the band structure with CHGCAR AND WAVECAR from step one using ISMEAR=1 because ISMEAR=-5 is not accepted in this case.
My question is :
Can I use the fermi energy from step 1 as my band structure fermi energy?
Obviously in my calculations, the smearing methods are different for these two steps.

Thanks a lot and Happy new year!

Decai
Last edited by decai on Mon Dec 31, 2007 2:07 am, edited 1 time in total.

admin
Administrator
Administrator
Posts: 2922
Joined: Tue Aug 03, 2004 8:18 am
License Nr.: 458

fermi energy in band structure calculation

#2 Post by admin » Thu Jan 03, 2008 11:09 am

The Fermi level is determined via the Kohn-Sham eigenvalue of the highest occupied state. Therefore please check from your band structure run if the corresponding k-point (state) was included in the kmesh you used for the runs done with ISMEAR=-5 at all.
please also mind that the absolute values of the KS-eigenvalues (and hence the Fermi levels) are only given to within a constant offset in energy, because the E=0 is not defined when 3D periodic boundary conditions are used. This may cause subtle differences in the absolute values of the KS energies in 2 different runs as well (a rough estimate can be obtained by comparing alpha+bet as written in OUTCAR).
Last edited by admin on Thu Jan 03, 2008 11:09 am, edited 1 time in total.

ralf_tonner1
Newbie
Newbie
Posts: 1
Joined: Mon Dec 21, 2020 2:19 pm

Re: fermi energy in band structure calculation

#3 Post by ralf_tonner1 » Fri Jul 16, 2021 3:39 pm

Dear admin,
Although this thread seems to be quite old but recently, I came across a similar problem again. As was mentioned in your comment that in different run absolute KS eigenvalues could be different due to difference in constant offset. However, I calculated bulk GaAs (3D periodic) band structure (along L-G-X-K U-G) with the same settings 20 times and every time I got the same absolute band energies at every k-points. The Fermi energies are exactly the same as well. So, I was wondering if I am misunderstanding something. If I am not wrong, we are basically solving this below Schrödinger equation. So, I was wondering in which part of the below equation the constant offset is contributing. If you don't mind I would like to ask for a little more clarification about the 'constant offset' part. Also, if possible providing some reference could be extremely helpful.
KS.jpg
KS2.jpg
KS3.jpg
You do not have the required permissions to view the files attached to this post.

henrique_miranda
Global Moderator
Global Moderator
Posts: 234
Joined: Mon Nov 04, 2019 12:41 pm
Contact:

Re: fermi energy in band structure calculation

#4 Post by henrique_miranda » Mon Jul 19, 2021 2:27 pm

This is indeed quite an old thread.
I don't know if I am still able to find the person that answered this question in the first place to explain what is meant. Unfortunately, different people could have answered under the admin alias.

I will try to explain it in my own words. Hopefully, this clarifies your doubts.

To the question of the user I would have simply have answered that yes, you can and should use the fermi energy from the first run because to determine the fermi-energy you need a regular grid.
Computing the fermi level from the band-structure run (i.e. only with k-points along a path in the BZ) while technically possible is meaningless. For hybrid functionals, the current recommendation is to use a regular grid + k-points along a path with zero weight:
wiki/index.php/Si_bandstructure#Procedu ... Hybrids.29

As for the discussion about the constant energy offset, I do not know what was meant originally.
The only thing that comes to mind is how the total energies in VASP are computed with respect to the isolated reference atoms that was used to generate the POTCAR:
wiki/index.php/Calculation_of_atoms#Det ... y_of_atoms
This would refer to the v_ext term in the equation you posted.

This being said, this is not something you need to worry about in this situation since you are not changing the POTCAR file between the SCF and band-structure calculation.

Post Reply