# [Pw_forum] comparison of ph.x and dynmat.x results

Elie M elie.moujaes at hotmail.co.uk
Mon Sep 10 20:48:23 CEST 2012

```Professor de Gironcoli,
First of all. thanks for your suggestion. Concerning moving the atoms along the modes, how can this be done? How do we know their directions? Vizualizing them in XCrysDen, for example,  would not help I guess because they will be set to zero

Elie

> Date: Sat, 8 Sep 2012 09:17:05 +0200
> From: degironc at sissa.it
> To: pw_forum at pwscf.org
> Subject: Re: [Pw_forum] comparison of ph.x and dynmat.x results
>
> negative (imaginary) frequencies signal instabilities.
> acoustic modes (i.e. rigid global translations on the crystal) at
> gamma should always have zero frequencies but for numerical reasons
> they can result in small positive or negative values that can be fixed
> by the acoustic sum rule.
>
> in your case the modee at -49, 50 and 73 are the acoustic modes that
> vanish after ASR inclusion.
> the modes around -370 are other modes and they are unstable..
>
> move the atoms of your structure along one of this modes and relax it again.
> this will probably break a symmetry that prevented your system to
> reach complete relaxation
>
> stefano
>
>
> Quoting Elie M <elie.moujaes at hotmail.co.uk>:
>
> > Dear all, I have done phonon calculations at the Gamma point to find
> > the vibrational frequencies of a system I am working on and I got
> > three negative frequencies; the results are:
> >  q = (    0.000000000   0.000000000   0.000000000 )
> >
> > **************************************************************************
> >   omega( 1) =     -11.303890 [THz] =    -377.057178 [cm-1]
> > omega( 2) =     -11.228798 [THz] =    -374.552397 [cm-1]     omega(
> > 3) =      -1.493780 [THz] =     -49.827127 [cm-1]     omega( 4) =
> >    1.499866 [THz] =      50.030148 [cm-1]     omega( 5) =
> > 2.192955 [THz] =      73.149113 [cm-1]     omega( 6) =
> > 11.690342 [THz] =     389.947822 [cm-1]     omega( 7) =
> > 15.929343 [THz] =     531.345701 [cm-1]     omega( 8) =
> > 17.762801 [THz] =     592.503255 [cm-1]     omega( 9) =
> > 17.814756 [THz] =     594.236307 [cm-1]     omega(10) =
> > 22.128875 [THz] =     738.139807 [cm-1]     omega(11) =
> > 24.754227 [THz] =     825.712121 [cm-1]     omega(12) =
> > 25.174421 [THz] =     839.728307 [cm-1]     omega(13) =
> > 25.229402 [THz] =     841.562251 [cm-1]     omega(14) =
> > 31.677488 [THz] =    1056.647257 [cm-1]     omega(15) =
> > 32.931458 [THz] =    1098.475192 [cm-1]     omega(16) =
> > 32.974208 [THz] =    1099.901170 [cm-1]     omega(17) =
> > 37.529033 [THz] =    1251.833794 [cm-1]     omega(18) =
> > 37.585396 [THz] =    1253.713860 [cm-1]     omega(19) =
> > 38.689108 [THz] =    1290.529726 [cm-1]     omega(20) =
> > 44.468725 [THz] =    1483.317012 [cm-1]     omega(21) =
> > 44.490793 [THz] =    1484.053106 [cm-1]     omega(22) =
> > 100.618488 [THz] =    3356.271501 [cm-1]     omega(23) =
> > 100.705119 [THz] =    3359.161186 [cm-1]     omega(24) =
> > 103.337467 [THz] =    3446.966862 [cm-1]
> > To check whether the first three freqnecies are the accoustic ones
> > and not instabilities  i applied dynmat.x with asr='crystal' and got:
> >   mode   [cm-1]     [THz]       IR    1   -377.06  -11.3039
> > 0.0000    2   -374.55  -11.2287    0.0000    3      0.00    0.0000
> >  0.0000    4      0.00    0.0000    0.0000    5      0.00    0.0000
> >   0.0000    6    406.11   12.1749    0.0000    7    531.35   15.9293
> >    0.0000    8    592.50   17.7628    0.0000    9    594.24
> > 17.8148    0.0000   10    738.13   22.1286    0.0000   11    828.85
> >  24.8482    0.0000   12    839.68   25.1730    0.0000   13    841.62
> >   25.2311    0.0000   14   1056.65   31.6775    0.0000   15
> > 1099.09   32.9498    0.0000   16   1099.25   32.9547    0.0000   17
> >  1251.32   37.5135    0.0000   18   1253.47   37.5781    0.0000   19
> >   1290.53   38.6891    0.0000   20   1483.05   44.4608    0.0000
> > 21   1485.30   44.5283    0.0000   22   3356.09  100.6131    0.0000
> >  23   3359.58  100.7178    0.0000   24   3446.96  103.3373    0.0000
> > As it is seen, the freqencies are very close but the thing and the
> > system is stable! Howevere,I could not understand is about the first
> > 5 frequencies: i still got the first two negative but in addition i
> > have 3 more zero frequencies; does it mean we have five accoustic
> > modes due to the symmetry of this particular system?
> >
> > Elie KoujaesUniversity of NottsNG7 2RDUK
>
>
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