[Pw_forum] comparison of ph.x and dynmat.x results
Stefano de Gironcoli
degironc at sissa.it
Sat Sep 8 09:17:05 CEST 2012
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?
>
> Thanks in advance
> Elie KoujaesUniversity of NottsNG7 2RDUK
More information about the users
mailing list