[Pw_forum] phonon eigenvectors

[Eric Abel]

Hello PWSCF users,

I have a question which is sort of a follow-up to a similar question which I 
asked previously.  Anyway, I was wondering about the imaginary part of the 
phonon eigenvectors, which at the time, Stefano had informed me are more or 
less arbitrary.   At the time that made sense.  But now I am trying to make 
sense of the following eigenvectors:

q =       0.0000      0.0563      0.0000
**************************************************************************
     omega( 1) =      -0.308036 [THz] =     -10.275037 [cm-1]
(  0.000000   0.000000     0.000000  -0.142674    -0.338669   0.000000   )
(  0.000000   0.000000     0.031918  -0.201463    -0.388896  -0.061613   )
(  0.000000   0.000000     0.019612  -0.123787    -0.367551  -0.058231   )
(  0.000000   0.000000     0.000000  -0.226888    -0.354765   0.000000   )
(  0.000000   0.000000     0.036445  -0.230039    -0.328252  -0.052005   )
(  0.000000   0.000000     0.000000  -0.096682    -0.403396   0.000000   )
     omega( 2) =       0.617574 [THz] =      20.600196 [cm-1]
( -0.403925  -0.012145     0.000000   0.000000     0.000000   0.000000   )
( -0.397050  -0.075201     0.000000   0.000000     0.000000   0.000000   )
( -0.397199  -0.075229     0.000000   0.000000     0.000000   0.000000   )
( -0.404073  -0.012149     0.000000   0.000000     0.000000   0.000000   )
( -0.408949  -0.077455     0.000000   0.000000     0.000000   0.000000   )
( -0.416112  -0.012511     0.000000   0.000000     0.000000   0.000000   )
     omega( 3) =       0.788199 [THz] =      26.291668 [cm-1]
(  0.000000   0.000000     0.000000   0.389819    -0.233273   0.000000   )
(  0.000000   0.000000    -0.055122   0.347926    -0.110454  -0.017499   )
(  0.000000   0.000000    -0.061881   0.390589    -0.148121  -0.023467   )
(  0.000000   0.000000     0.000000   0.342277    -0.187111   0.000000   )
(  0.000000   0.000000    -0.050913   0.321357    -0.249835  -0.039581   )
(  0.000000   0.000000     0.000000   0.384275    -0.092029   0.000000   )

These are the first 3 (accoustic) modes of the spectrum.  If the imaginary 
part doesn't matter, then it would appear that the accoustic displacement 
along the b-direction is practically zero.  However, the imaginary part is 
the same magnitude as the displacements in the other orthogonal directions.  
Is there any physical meaning to this, or is it just an artifact of the 
calculation?

Eric,
Ph.D. Student, M.I.T.