# [Pw_forum] why K-X point of phonon dispersion is wrong?

lucking-pine songsong19840614 at gmail.com
Sat May 14 03:04:44 CEST 2011

Dear Eyvaz Isaev:
And I want to ask that  I use these k-points into <band.input>,and then the
<band.output>will auto-transform the k-point into  Cartesian and units of
2*\pi/a,and I use these output k-point in <matdyn.input>.I always get
k-point in this way.Is there something wrong?

Bests,
Pine.

2011/5/14 Eyvaz Isaev <eyvaz_isaev at yahoo.com>

>  Hi,
>
> XCryDen gives you  q-points coordinates with respect to basis vectors.
> But X(1,1,0) and K(3/4,3/4,0) are in Cartesian and in units of 2*\pi/a.
>
> If I am not wrong, matdyn.x uses (at least, always I used)  q-points in
> Cartesian, so, you have to transform these points to Cartesian.
> Or see a text book  (H.Jones The Theory of Brillouin zones and electronic
> states in crystals, or J. Callaway Energy band Theory, or
> whatever you like)   to find them in Cartesian.
>
>
> Bests,
> Eyvaz.
>
>  ------------------------------
> *From:* lucking-pine <songsong19840614 at gmail.com>
> *To:* PWSCF Forum <pw_forum at pwscf.org>
> *Sent:* Wed, May 11, 2011 5:34:10 AM
> *Subject:* Re: [Pw_forum] why K-X point of phonon dispersion is wrong?
>
> Dear Eyvaz Isaev:
> Thank you for your advice.My k-point is chosen by Xcrysden.such as:
> G   0.0000000000     0.0000000000     0.0000000000
> K  -0.7500000000    -0.3750000000    -0.3750000000
> X  -0.5000000000     0.0000000000    -0.5000000000
> G  0.0000000000     0.0000000000     0.0000000000
> L - 0.5000000000    -0.5000000000    -0.5000000000
> X   0.0000000000    -0.5000000000    -0.5000000000
> W -0.2500000000    -0.5000000000    -0.7500000000
> L  -0.5000000000    -0.5000000000    -0.5000000000
>
> and I try to use your advice through using 3 type of attempt.
> 1.only chang K,Xpoint of K-G path to K(0.75,0.75,0), X point (1, 1, 0)
> 2.all k point multipy 2 times.
>    such as:
>
> G   0.0000000000     0.0000000000     0.0000000000
> K  -1.5000000000    -0.7500000000    -0.7500000000
> X  -1.0000000000     0.0000000000    -1.0000000000
> G  0.0000000000     0.0000000000     0.0000000000
> L - 1.0000000000    -1.0000000000    -1.0000000000
> X   0.0000000000    -1.0000000000    -1.0000000000
> W -0.5000000000    -1.0000000000    -1.5000000000
> L  -1.0000000000    -1.0000000000    -1.0000000000
>
> 3.only K,Xpoint of K-G path multiply 2 times.
>    such as
>
> G   0.0000000000     0.0000000000     0.0000000000
> K  -1.5000000000    -0.7500000000    -0.7500000000
> X  -1.0000000000     0.0000000000    -1.0000000000
> G  0.0000000000     0.0000000000     0.0000000000
> L - 0.5000000000    -0.5000000000    -0.5000000000
> X   0.0000000000    -0.5000000000    -0.5000000000
> W -0.2500000000    -0.5000000000    -0.7500000000
> L  -0.5000000000    -0.5000000000    -0.5000000000
>
> However,all this attempts yield wrong picture. So,this must be something I
> didn't understand correctly.Would you tell me where is my fault ?
> 2011/5/6 Eyvaz Isaev <eyvaz_isaev at yahoo.com>
>
>>  Hi,
>>
>> First of all I would recommend  the use of PlotPhon utility to plot phonon
>> dispersion relations. For this one you have to have calculated interatomic
>> force constants matrix (*.fc) file.
>>
>> Regarding your particular question: you should choose K(0.75,0.75,0), then
>> from the  K point go to the X point (1, 1, 0) which is equivalent to
>> the X(1,0,0) point. Then from the X(1,0,0) to the Gamma (0,0,0) point.
>> Dashed line just means that we can continue in this direction to match
>> phonons at the X point (i.e. it is an intermediate point from the Gamma to
>> the X point) .
>>
>>
>> Bests,
>> Eyvaz
>>
>> -------------------------------------------------------------------
>> Prof. Eyvaz Isaev,
>> Department of Physics, Chemistry, and Biology (IFM), Linkoping University,
>> Sweden
>> Theoretical Physics Department, Moscow State Institute of Steel & Alloys,
>> Russia,
>> isaev at ifm.liu.se, eyvaz_isaev at yahoo.com
>>
>>
>>  ------------------------------
>> *From:* lucking-pine <songsong19840614 at gmail.com>
>> *To:* Pw_forum at pwscf.org
>> *Sent:* Fri, May 6, 2011 2:50:16 PM
>> *Subject:* [Pw_forum] why K-X point of phonon dispersion is wrong?
>>
>> My pwscf is used in calculating Si phonon dispersion.The trend of picture
>> in
>>
>> most special Symmetry is right.But the only K-X is wrong.I use Xcrysden to
>>
>>
>> get K-point.
>> I found most paper is dash line in 'K' point.So what is mean? Please
>> anyone
>>
>> could tell me!  Thanks!
>> The attachment is my picture and reference paper.
>>
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>> Pw_forum at pwscf.org
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>>
>>
>
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