[Pw_forum] [SOLVED ?] Re: weird phonon frequencies for 3X 3X 1 KPOINTS

[Gabriele Sclauzero]

On 09/30/2010 08:47 PM, Mehmet Topsakal wrote:
> Dear Gabriele,
>
> Thank you very much for your suggestion. After including some 
> smearing, -5000 frequencies disappeared. But this
> still cannot explain the 3X 3X 1 situation.

Why is this situation so special? BTW, I cannot find any result about 
this case in your repository.

> For this reason, I plotted the positions of scf KPOINTS (see the 
> attachments)
> and realized that the corner of the Brillouin zone, which is also 
> known as K-special-point, is NOT included for KPOINTS
> other than 3X 3X 1. For graphene, K-special-point also corresponds to 
> the Dirac-Point where zero-band-gap occurs.

 From your nice drawing of the k-points, I guess that the "Dirac" 
K-point is included also in the 9x9x1 and 12x12x1 meshes (without shift, 
at least that's what your have written in the picture and in the input 
files), so this may explain why you get strange results for those meshes 
without smearing.

> As a result, problem should be related with K-special-point.

If you include the shift I think that the K-point will never be included 
(excepting the 3x3x1 grid perhaps). I suggest you to study the 
convergence of the frequencies separately for the shifted and unshifted 
grids (both with smearing) and see if they converge to the same result. 
Some people believe that including the K-point in the BZ sampling is of 
vital importance, even with dense meshes. I'm not sure of that.

>
> BUT. Still I cannot understand why inclusion of a point in BZ 
> corresponds to such a HUGE difference in phonon frequencies.
> Maybe someone in this list gives a better explanation for that....

I cannot explain this, sorry. Let's wait for an answer from a graphene 
or phonon expert (or both).


Regards


GS

>
> All of the files are available at http://db.tt/xK2Jtu1
>
> Ragards.
>
>
> On Thu, Sep 30, 2010 at 4:47 PM, Gabriele Sclauzero <sclauzer at sissa.it 
> <mailto:sclauzer at sissa.it>> wrote:
>
>     Dear Mehmet,
>
>        graphene has zero gap, hence you probably need to include
>     smearing of the occupations in the scf calculations.
>
>
>     HTH
>
>     GS
>
>
>     On 09/29/2010 04:54 PM, Mehmet Topsakal wrote:
>>     Dear all,
>>
>>     For graphene structure, I noticed that I get completely wrong
>>     phonon frequencies when I use KPOINTS such as 9 9 1, 12 12 1, 15 15 1
>>
>>     For example:
>>
>>     K_POINTS {automatic}
>>     11 11 1    0 0 0
>>
>>     gives realistic frequencies as in 11.phonon.out file
>>
>>     BUT
>>
>>     K_POINTS {automatic}
>>     12 12 1    0 0 0
>>
>>     gives "omega( 1) =    -149.592762 [THz] =   -4989.910586 [cm-1]"
>>     as in 12.phonon.out
>>
>>
>>     What should be the reason ????
>>
>>     Thanks.
>>
>>     -- 
>>
>>     Mehmet Topsakal  (Ph.D. Student)
>>     UNAM-Institute of Materials Science and Nanotechnology.
>>     Bilkent University. 06800 Bilkent, Ankara/Türkiye
>>     Tel: 0090 312 290 3527 ; Fax: 0090 312 266 4365
>>     UNAM-web  : www.nano.org.tr <http://www.nano.org.tr>
>>
>>
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>
>
>     -- 
>
>     Gabriele Sclauzero, EPFL SB ITP CSEA
>     PH H2 462, Station 3, CH-1015 Lausanne
>          
>
>
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>
>
>
> -- 
>
> Mehmet Topsakal  (Ph.D. Student)
> UNAM-Institute of Materials Science and Nanotechnology.
> Bilkent University. 06800 Bilkent, Ankara/Türkiye
> Tel: 0090 312 290 3527 ; Fax: 0090 312 266 4365
> UNAM-web  : www.nano.org.tr <http://www.nano.org.tr>
>
>
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-- 

Gabriele Sclauzero, EPFL SB ITP CSEA
PH H2 462, Station 3, CH-1015 Lausanne

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