[Pw_forum] Bls: Bls: Relax Calculation for ZnO

[Angga Fauzi]

Dear Giuseppe,

Thank you for your answer. It is very helpful for me. I will use your advice in my calculation. :-)

Regards,
Angga


 
Angga Dito Fauzi
Undergraduate Student
Department of Physics, Faculty of Mathematics and Natural Sciences
Universitas Indonesia, Depok 16424, Indonesia
Phone: +628124139348
Email: angga_dito_fauzi at yahoo.com


________________________________
 Dari: Giuseppe Mattioli <giuseppe.mattioli at ism.cnr.it>
Kepada: Angga Fauzi <angga_dito_fauzi at yahoo.com>; PWSCF Forum <pw_forum at pwscf.org> 
Dikirim: Kamis, 10 Januari 2013 18:54
Judul: Re: [Pw_forum] Bls:  Relax Calculation for ZnO
 

Dear Angga
A 32 atoms ZnO cells is not so big. I recommend you to perform a bunch of different calculations in 
order to touch with your hands the difference between fixed occupations, smearing methods, ...
But let us say that in the case of a ZnO bulk you should not find differences between fixed and 
smearing (start always with Gaussian smearing: it improves the scf convergence) calculations because 
an intrinsic semiconductor is in fact an insulator: No electron (or significant fraction of electron) 
will cross the band gap and populate the conduction band, even if the GGA ZnO gap is generally 
reduced to the quite low value of about 1.0 eV. You can check it by the smearing contribution to the 
total energy, which will be negligible. When you dope the ZnO lattice with a TM atom (a 32 atoms 
cell is too small: the TM atom will strongly interact with its periodically repeated image) the 
system could turn out to be a semiconductor, that is, technically speaking, a "metal" with the Fermi 
energy crossing some bands. Here you must use smearing, or you will never reach the scf convergence. 
I always use a low Gaussian smearing (0.01 Ry) to relax this kind of system. After relaxing the 
system, you can try to restart from the final positions by using different smearing values and kinds 
(mv, fd, ...) and check if the results are the same (usually they are!). You do not need to perform 
a scf calculation after the relax one to calculate the band structure, unless you are performing EXX 
calculations which do not allow for the nscf procedure. In the case of vc-relax calculations, the 
code itself performs a final, unbiased, scf calculation because of the change of the k-points mesh 
along the vc-relax procedure, but this is not necessary in the case of simple relax calculations.

HTH

Giuseppe


On Thursday 10 January 2013 12:16:59 Angga Fauzi wrote:
> Dear Nazari,
> 
> Thank you for your answer. I want to ask again.
> 
>     1. According to your answer for my first question, since my system is ZnO,
> which is a semiconductor, then I use occupations = 'fixed'. How if my
> system is TM-doped ZnO? Should I use occupations = 'fixed' again or I
> replace it with occupations = 'smearing'? 2. According to your answer for
> my second question, is it means that after my relax calculation is done, I
> can directly run nscf calculation to calculate band structure or I have to
> run scf calculation first and then nscf calculation? Regards,
> Angga
> 
> Angga Dito Fauzi
> Undergraduate Student
> Department of Physics, Faculty of Mathematics and Natural Sciences
> Universitas Indonesia, Depok 16424, Indonesia
> Phone: +628124139348
> Email: angga_dito_fauzi at yahoo.com
> 
> 
> ________________________________
>  Dari: "nazari at iasbs.ac.ir" <nazari at iasbs.ac.ir>
> Kepada: Angga Fauzi <angga_dito_fauzi at yahoo.com>; PWSCF Forum
> <pw_forum at pwscf.org> Dikirim: Kamis, 10 Januari 2013 15:18
> Judul: Re: [Pw_forum] Relax Calculation for ZnO
> 
> > Dear all QE users,
> > 
> > I am trying to run relax
> 
> calculation for my ZnO system. There are
> 
> > something I want to
> 
> ask.
> 
> > 1. In running relax calculation, what occupations should I
> 
> use? I am
> 
> > trying to use occupations = 'tetrahedra', but it
> 
> doesn't work. I am
> 
> > afraid if I use occupations = 'smearing', my
> 
> result calculation doesn't
> 
> > valid since ZnO is a semiconductor,
> 
> not a metal. 
> 
>   For semiconductor  
> occupations=fixed
> 
> > 2. After my relax calculation is
> 
> done, should I run a scf calculation
> 
> > again with atomic
> 
> positions I get from relax calculation? 
>  
>    For what?
>  
> 
> > I also attach my
> 
> input file. Thank you very much for your attention.
> 
> 
> Regards,
> 
> > Angga
> >  
> > Angga Dito Fauzi
> > Undergraduate Student
> > Department of Physics, Faculty
> 
> of Mathematics and Natural Sciences
> 
> > Universitas Indonesia,
> 
> Depok 16424, Indonesia
> 
> > Phone: +628124139348
> 
> > Email:
> angga_dito_fauzi at yahoo.com
> scanned for viruses and
> 
> > dangerous content by MailScanner, and
> 
> is
> 
> > believed to be clean.
> 
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> 
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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA  
   v. Salaria Km 29,300 - C.P. 10                
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