[Pw_forum] ASR in an unstable structure

[Uri Argaman]

Dear QE users and developers
I run a phonon calculation in SnSe (Roch-salt structure) and I get the
frequencies in the Gamma point (dyn1 file):
     q = (    0.000000000   0.000000000   0.000000000 )
 **************************************************************************
     freq (    1) =      -0.947936 [THz] =     -31.619734 [cm-1]
 (  0.463998  0.000000 -0.152295  0.000000 -0.248340  0.000000 )
 ( -0.708498  0.000000  0.232546  0.000000  0.379201  0.000000 )
     freq (    2) =      -0.947936 [THz] =     -31.619734 [cm-1]
 ( -0.267553  0.000000 -0.407542  0.000000 -0.249970  0.000000 )
 (  0.408539  0.000000  0.622293  0.000000  0.381690  0.000000 )
     freq (    3) =      -0.947936 [THz] =     -31.619734 [cm-1]
 (  0.115246  0.000000 -0.332981  0.000000  0.419526  0.000000 )
 ( -0.175974  0.000000  0.508442  0.000000 -0.640593  0.000000 )
     freq (    4) =      -0.098311 [THz] =      -3.279313 [cm-1]
 ( -0.486385  0.000000  0.205881  0.000000  0.478423  0.000000 )
 ( -0.478812  0.000000  0.202675  0.000000  0.470973  0.000000 )
     freq (    5) =      -0.098311 [THz] =      -3.279313 [cm-1]
 (  0.492088  0.000000  0.396133  0.000000  0.329809  0.000000 )
 (  0.484426  0.000000  0.389965  0.000000  0.324673  0.000000 )
     freq (    6) =      -0.098311 [THz] =      -3.279313 [cm-1]
 (  0.170660  0.000000 -0.555462  0.000000  0.412533  0.000000 )
 (  0.168003  0.000000 -0.546813  0.000000  0.406109  0.000000 )
 **************************************************************************
After I use q2r.x and plotphonon to plot the spectrum (using matdyn.x with
asr='crystal') I get the frequencies in the Gamma point: 0,0,0 (for
acoustic, which is good) and ~ -1 -1 3 THz for the optics ,which is
strange. From where the 3 THz frequency come from? Which frequencies are
more reliable, the original ones or those of matdyn?
I note that it is probably unstable structure but I still want to now what
are the unstable modes.

Thank you very much
Uri Argaman
Ben-Gurion University
Israel
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