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

Eyvaz Isaev eyvaz_isaev at yahoo.com
Fri May 13 22:33:52 CEST 2011

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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>
>
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