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

[Elie M]

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
 		 	   		  
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