Yes, I thought that the non zero negative frequencies that remained still signal instability. Will do what you suggested. Thanks
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? > > > > Thanks in advance > > Elie KoujaesUniversity of NottsNG7 2RDUK > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20120908/5bb024a5/attachment.htm
