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