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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