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<p>Dear José,</p>
<p><br>
</p>
<p>what is the Fermi energy? Well, at 0K it tells you that all states below are occupied and all states above are unoccupied.<br>
Yet, if you have a band gap, as in your case, the Fermi energy could be anywhere in the band gap and this wouldn't<br>
change a thing - standard DFT is for 0K and the broadening used in the code (which is only needed for metals) is a<br>
numerical trick to help with the convergence. If people plot the band structure for a semiconductor w.r.t. the Fermi</p>
<p>energy, the result can depend on all possible things in the calculation and this is what you see - for one specific</p>
<p>k-point mesh you probably sample the valence band better and thus the Fermi energy is closer to the valence-band<br>
maximum. Again, for a semiconductor, the Fermi energy can be anywhere in the band gap and this does not change<br>
the result. In principle you don't need broadening for a semiconductor and then the code will print the HOMO and<br>
LUMO energies. You should plot the band structure to understand where is the position of the valence-band</p>
<p>maximum and then you will understand why 10x10x10 is different...</p>
<p><br>
</p>
<p>Regards</p>
<p><br>
</p>
<p>Thomas<br>
</p>
<p><br>
</p>
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<pre class="moz-signature" cols="72"><font size="3">--<br>Dr. rer. nat. Thomas Brumme<br>Theoretical chemistry<br>TU Dresden - KOE / 103<br><span id="ms-rterangepaste-start"></span><span>Bergstr. 66c</span><span id="ms-rterangepaste-end"></span><br>01069 Dresden<br><br>Tel: +49 (0)351 463 40844<br><br><font color="000000">email: thomas.brumme@tu-dresden.de</font></font><br></pre>
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<div id="x_divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>Von:</b> users <users-bounces@lists.quantum-espresso.org> im Auftrag von José Xavier via users <users@lists.quantum-espresso.org><br>
<b>Gesendet:</b> Dienstag, 4. Januar 2022 20:47<br>
<b>An:</b> Quantum ESPRESSO users Forum; Paolo Giannozzi; Kazume NISHIDATE<br>
<b>Betreff:</b> Re: [QE-users] Assunto: Re: Assunto: Re: Difference in Fermi energy by increasing k-points</font>
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<div class="PlainText">Dear Giannozzi,<br>
<br>
Yes, the bandgap and peaks are the same with a shift in the axis x. <br>
However, all the tutorials I've watched and read showed me that, by using the Efermi to set the zero in the graph, the last peak of the valence band should end in the zero (as in my reference). But, in my case, it only occurs when I use 10x10x10 in the NSCF
calculation and all other combinations of k-points gave me the second figure.<br>
<br>
It is strange, for me, that only one value of k-points showed me the behavior that I was expecting, and that is the reason for my question. What is happening in my calculations to make the k-point 10x10x10 the only one that shows me a value of EFermi that corresponds
to the "real" zero of the graph?<br>
<br>
Best wishes<br>
José Xavier<br>
<br>
<br>
<br>
Em terça-feira, 4 de janeiro de 2022 06:45:32 BRT, Paolo Giannozzi <p.giannozzi@gmail.com> escreveu:
<br>
<br>
<br>
<br>
<br>
<br>
The two figures are exactly the same, only the origin of the x axis is shifted.<br>
<br>
Paolo<br>
<br>
On Tue, Jan 4, 2022 at 10:29 AM José Xavier via users <users@lists.quantum-espresso.org> wrote:<br>
> Dear Nishidate,<br>
> <br>
> Thank you for your answer.<br>
> <br>
> I don't know if I understood... Could you explain me better what should I do, please?<br>
> <br>
> I'm sending a figure of my DOS plotted using the HOMO and the EFermi as the zero, respectively, to make my doubt clearer. <br>
> <br>
> I learned that I should use a "large" number of k-points to converge the system. However, I only found the first figure when I used the 10x10x10 k-points in the NSCF calculation, and 2x2x2, 4x4x4 or 6x6x6 in the SCF. When I increased or decreased the k-points
in the NSCF, it showed me the second figure (that is different from the reference one).<br>
> <br>
> I tried to use the smearing occupations. In this test, the EFermi in SCF and NSCF calculations were the same (2.74 eV), except when I used 10x10x10 k-points in the NSCF. In this case, the EFermi was 2.74 in SCF and 1.04 eV in the NSCF (it is the same value
that I found previously as the HOMO in SCF calculation). <br>
> <br>
> My main question is: Why is this 10x10x10 a "magic number"? <br>
> <br>
> Best wishes.<br>
> I wish you a happy new year.<br>
> <br>
> José Xavier<br>
> <br>
> <br>
>> <br>
>> <br>
>> Em qua., 29 29e dez. 29e 2021 às 18:22, Kazume NISHIDATE<br>
>> <nisidate@iwate-u.ac.jp> escreveu:<br>
> <br>
> <br>
>> <br>
>> <br>
>> <br>
>> <br>
> <br>
> <br>
> <br>
> Dear Xavier<br>
> <br>
>> When I used the k-points 4x4x4 (SCF) and 8x8x8 (NSCF), the HOMO and<br>
>> EFermi were different, and using EFermi to subtract the values in the<br>
> <br>
> If the k-point mesh you selected covers the typical energy peaks of<br>
> the valence band maximum (VBM or the HOMO in your description) depends<br>
> on the shape of Brillouin zone (BZ) of your system.<br>
> <br>
> You should try to plot the band structure and assess the VBM and find<br>
> out its k-point location. If the k-point mesh you selected does not<br>
> cover the k-point, then the VBM peak will not contribute to your DOS<br>
> <br>
> plot.<br>
> <br>
> <br>
> <br>
> <br>
> <br>
> 敬具 西館<br>
> <br>
> best regards<br>
> <br>
> <br>
> <br>
> nisidate@iwate-u.ac.jp<br>
> <br>
> kazume.nishidate@gmail.com<br>
> <br>
> _______________________________________________<br>
> Quantum ESPRESSO is supported by MaX (<a href="http://www.max-centre.eu" id="LPlnk353509" previewremoved="true">www.max-centre.eu</a>)<br>
> users mailing list users@lists.quantum-espresso.org<br>
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https://lists.quantum-espresso.org/mailman/listinfo/users</a><br>
<br>
<br>
-- <br>
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,<br>
Univ. Udine, via delle Scienze 206, 33100 Udine, Italy<br>
Phone +39-0432-558216, fax +39-0432-558222<br>
<br>
<br>
<br>
_______________________________________________<br>
Quantum ESPRESSO is supported by MaX (<a href="http://www.max-centre.eu" id="LPlnk802400" previewremoved="true">www.max-centre.eu</a>)<br>
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