[Pw_forum] Fermi energy from scf calculation

[Tram Bui]

Dear Gabriele,
      Thank you very much for your respond. The reason i'm interested in
the fermi energy is because I have done the DOS calculation for my bulk SiC
and defected SiC system. I want to plot DOS vs energy to see how the band
gap changes from bulk to defected SiC, and the Ef is used as a reference
point for the plot. Also, I'm trying to calculate the band structure of
bulk SiC and the process starts from scf calculation, then using the fermi
energy obtained there, to do the bands.x calculation. I noticed that when I
use "smearing" and "degauss" value (only this will give me the info on
Fermi energy in the output, is that correct?), for the calculation with
ibrav=0 (8 atoms listed, for a 1x1x1 zinc blend structure), the Fermi
energy is around 9.3 eV. While with the same "smearing" and "degauss", but
ibrav=2 (2 atoms listed, for a 1x1x1 zinc blend structure), the Ef is ~10.4
eV. I just found it is odd that I got different Ef with the same "degauss"
value but different ibrav. The structure and parameters for both ibrav's
used, are the same. Would you give me some insight information on why it
was the case?

Regards,
Tram

On Wed, Nov 23, 2011 at 2:08 AM, Gabriele Sclauzero <
gabriele.sclauzero at epfl.ch> wrote:

> Dear Tram Bui (again:),
>
>     please try to read some basic introductions about smearing techniques
> (also called "Broadening of electronic occupations") and understand what
> they do and what are they  meant for. Your material (bulk SiC) is an
> insulator (or a semiconductor, if you want) and thus has a well defined
> band gap. In principle you are not required to use smearing to study this
> material. The Fermi energy for an insulator is not well defined, strictly
> speaking, why are you interested in it? If you just want to compute the
> band gap there are other methods (a well converged DOS or a wise glance at
> the band structure).
> If for some reason you need to use smearing, of course your results will
> differ from those obtained with fixed occupations. The larger degauss, the
> larger the difference, as you might have already observed. The value that
> you will chose should depend on the desired target accuracy in the total
> energy (or other quantities you might want to look at), and on the purpose
> for which you introduce degauss... if there's no reason, well, then don't
> use smearing and your problem will vanish.
>
>
> Cheers,
>
> GS
>
>
>
> Il giorno 22/nov/2011, alle ore 22.50, Tram Bui ha scritto:
>
> Dear Ibrahim,
>    Thank you so much for your respond. I have done this with my SiC
> system. however, I wasn't sure how to obtain a reasonable degauss values
> for SiC, zincblend structure. I have done with couple values, 0.01 and
> 0.001. And I got different results for my energy. So would you please allow
> me to ask you another question regarding how one can obtain the suitable
> degauss value for their material, in my case, the cubic SiC ?
>
> Best Regards,
> Tram Bui
>
>
>
> On Thu, Oct 27, 2011 at 9:29 PM, bamidele ibrahim <
> bamideleibrahim at yahoo.com> wrote:
>
>>  Dear Tram,
>>   For you to get fermi energy from your scf calculation, you need to set
>> you 'occupation=smearing', and they
>> the type of smearing you want to use follow with 'degauss value'. With
>> all this in place you will get the fermi
>> energy.
>>
>> Regards,
>>
>> Adetunji Bamidele Ibrahim
>> Department of physics,University of Agriculture,
>> Abeokuta, Ogun State,Nigeria.
>>  ------------------------------
>> *From:* Tram Bui <trambui at u.boisestate.edu>
>> *To:* PWSCF Forum <pw_forum at pwscf.org>
>> *Sent:* Thursday, October 27, 2011 11:05 PM
>> *Subject:* [Pw_forum] Fermi energy from scf calculation
>>
>>  Dear Everyone,
>>      I have told that the scf calculation would give out the Fermi
>> energy, Ef, of a material system. I have done some nscf, relax and vc-relax
>> calculations for the same material as well. I was able to obtain the Ef on
>> other calculation but not on the scf calculation. would you give me some
>> information on why it was the case?
>>
>> Regards,
>>
>> Tram Bui
>>
>> M.S. Materials Science & Engineering
>> trambui at u.boisestate.edu
>>
>>
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>
>
> --
> Tram Bui
>
> M.S. Materials Science & Engineering
> trambui at u.boisestate.edu
>
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>
> § Gabriele Sclauzero, EPFL SB ITP CSEA
> *   PH H2 462, Station 3, CH-1015 Lausanne*
>
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-- 
Tram Bui

M.S. Materials Science & Engineering
trambui at u.boisestate.edu
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