[Pw_forum] Confused about vibrational modes of isolated molecule from ph.x

[Jin Zhang]

Dear all,

I've tested the supercell approach using ph.x to calculate the vibrational
frequency of a benzene molecule.  It works indeed. And I got 36(12*3) eigen
values.

The thing I cannot understand is, there're 12 negative frequencies existing.
I know 3 of them are purely translational modes and another 3 are rotational
ones. But what about the rest 6? I'm a little confused and hopefully someone
can help.

The eigenvals are pasted below and thanks in advance!

(the result may not be accurate due to a only-15Ry ecut I used, but it
should not give out qualitative different result)
     omega( 1) =     -75.772470 [THz] =   -2527.514330 [cm-1]
     omega( 2) =     -70.357356 [THz] =   -2346.884371 [cm-1]
     omega( 3) =     -69.734785 [THz] =   -2326.117499 [cm-1]
     omega( 4) =     -60.745067 [THz] =   -2026.250783 [cm-1]
     omega( 5) =     -60.633397 [THz] =   -2022.525849 [cm-1]
     omega( 6) =     -54.624392 [THz] =   -1822.085685 [cm-1]
     omega( 7) =     -53.629434 [THz] =   -1788.897259 [cm-1]
     omega( 8) =     -49.221716 [THz] =   -1641.870640 [cm-1]
     omega( 9) =     -48.767076 [THz] =   -1626.705379 [cm-1]
     omega(10) =     -45.544107 [THz] =   -1519.197979 [cm-1]
     omega(11) =     -40.582868 [THz] =   -1353.707753 [cm-1]
     omega(12) =     -29.501111 [THz] =    -984.057687 [cm-1]
     omega(13) =      17.297930 [THz] =     577.000669 [cm-1]
     omega(14) =      18.430268 [THz] =     614.771652 [cm-1]
     omega(15) =      18.978064 [THz] =     633.044293 [cm-1]
     omega(16) =      21.573348 [THz] =     719.614207 [cm-1]
     omega(17) =      21.661957 [THz] =     722.569926 [cm-1]
     omega(18) =      22.212850 [THz] =     740.945851 [cm-1]
     omega(19) =      26.603080 [THz] =     887.389115 [cm-1]
     omega(20) =      27.163155 [THz] =     906.071359 [cm-1]
     omega(21) =      28.988845 [THz] =     966.970217 [cm-1]
     omega(22) =      29.897145 [THz] =     997.268046 [cm-1]
     omega(23) =      30.039795 [THz] =    1002.026353 [cm-1]
     omega(24) =      34.221037 [THz] =    1141.498514 [cm-1]
     omega(25) =      35.556617 [THz] =    1186.048959 [cm-1]
     omega(26) =      41.365733 [THz] =    1379.821484 [cm-1]
     omega(27) =      48.259770 [THz] =    1609.783366 [cm-1]
     omega(28) =      51.789502 [THz] =    1727.523303 [cm-1]
     omega(29) =      51.871548 [THz] =    1730.260091 [cm-1]
     omega(30) =      55.413486 [THz] =    1848.407220 [cm-1]
     omega(31) =      85.179130 [THz] =    2841.288830 [cm-1]
     omega(32) =      87.544947 [THz] =    2920.204497 [cm-1]
     omega(33) =      88.051107 [THz] =    2937.088312 [cm-1]
     omega(34) =      89.398003 [THz] =    2982.016222 [cm-1]
     omega(35) =      93.664201 [THz] =    3124.322195 [cm-1]
     omega(36) =      95.571010 [THz] =    3187.926929 [cm-1]
-- 
Jin Zhang
Dep. of Physics,
Peking Univ.
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