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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:<div><br></div><div><div> q = ( 0.000000000 0.000000000 0.000000000 )</div><div><br></div><div> **************************************************************************</div><div> omega( 1) = -11.303890 [THz] = -377.057178 [cm-1]</div><div> omega( 2) = -11.228798 [THz] = -374.552397 [cm-1]</div><div> omega( 3) = -1.493780 [THz] = -49.827127 [cm-1]</div><div> omega( 4) = 1.499866 [THz] = 50.030148 [cm-1]</div><div> omega( 5) = 2.192955 [THz] = 73.149113 [cm-1]</div><div> omega( 6) = 11.690342 [THz] = 389.947822 [cm-1]</div><div> omega( 7) = 15.929343 [THz] = 531.345701 [cm-1]</div><div> omega( 8) = 17.762801 [THz] = 592.503255 [cm-1]</div><div> omega( 9) = 17.814756 [THz] = 594.236307 [cm-1]</div><div> omega(10) = 22.128875 [THz] = 738.139807 [cm-1]</div><div> omega(11) = 24.754227 [THz] = 825.712121 [cm-1]</div><div> omega(12) = 25.174421 [THz] = 839.728307 [cm-1]</div><div> omega(13) = 25.229402 [THz] = 841.562251 [cm-1]</div><div> omega(14) = 31.677488 [THz] = 1056.647257 [cm-1]</div><div> omega(15) = 32.931458 [THz] = 1098.475192 [cm-1]</div><div> omega(16) = 32.974208 [THz] = 1099.901170 [cm-1]</div><div> omega(17) = 37.529033 [THz] = 1251.833794 [cm-1]</div><div> omega(18) = 37.585396 [THz] = 1253.713860 [cm-1]</div><div> omega(19) = 38.689108 [THz] = 1290.529726 [cm-1]</div><div> omega(20) = 44.468725 [THz] = 1483.317012 [cm-1]</div><div> omega(21) = 44.490793 [THz] = 1484.053106 [cm-1]</div><div> omega(22) = 100.618488 [THz] = 3356.271501 [cm-1]</div><div> omega(23) = 100.705119 [THz] = 3359.161186 [cm-1]</div><div> omega(24) = 103.337467 [THz] = 3446.966862 [cm-1]</div><div><br></div><div>To check whether the first three freqnecies are the accoustic ones and not instabilities i applied dynmat.x with asr='crystal' and got:</div><div><br></div><div><div> mode [cm-1] [THz] IR</div><div> 1 -377.06 -11.3039 0.0000</div><div> 2 -374.55 -11.2287 0.0000</div><div> 3 0.00 0.0000 0.0000</div><div> 4 0.00 0.0000 0.0000</div><div> 5 0.00 0.0000 0.0000</div><div> 6 406.11 12.1749 0.0000</div><div> 7 531.35 15.9293 0.0000</div><div> 8 592.50 17.7628 0.0000</div><div> 9 594.24 17.8148 0.0000</div><div> 10 738.13 22.1286 0.0000</div><div> 11 828.85 24.8482 0.0000</div><div> 12 839.68 25.1730 0.0000</div><div> 13 841.62 25.2311 0.0000</div><div> 14 1056.65 31.6775 0.0000</div><div> 15 1099.09 32.9498 0.0000</div><div> 16 1099.25 32.9547 0.0000</div><div> 17 1251.32 37.5135 0.0000</div><div> 18 1253.47 37.5781 0.0000</div><div> 19 1290.53 38.6891 0.0000</div><div> 20 1483.05 44.4608 0.0000</div><div> 21 1485.30 44.5283 0.0000</div><div> 22 3356.09 100.6131 0.0000</div><div> 23 3359.58 100.7178 0.0000</div><div> 24 3446.96 103.3373 0.0000</div></div><div><br></div><div>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?</div><div><br></div><div><br></div><div>Thanks in advance</div><div><br></div><div>Elie Koujaes</div><div>University of Notts</div><div>NG7 2RD</div><div>UK</div><div><br></div></div> </div></body>
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