[QE-users] the difference bettewn PH and phonopy

[Zeli Xu]

Dear QE users and developers,
  I have some questions about phonon calculations:
  1): what is the unit of the displacement vector in *dyn* files? I think the displacement vector is under the "1 1" as shown below. 

######################################################################
  Dynamical  Matrix in cartesian axes

     q = (    0.000000000  -1.000000000   0.000000000 )

    1    1
  0.24766291   0.00000000    -0.00000000   0.00000000     0.00000000   0.00000000
  0.00000000   0.00000000     0.35585327   0.00000000     0.00000000   0.00000000
  0.00000000   0.00000000     0.00000000   0.00000000     0.24766291   0.00000000
######################################################################

  2): in the output file matdyn.modes as shown below. There are also some vectors that are similar with the displacement vector in *dyn* files. The vector is under the "freq (    1) = -0.000000 [THz] = -0.000000 [cm-1]". But obviously, the vectors in file matdyn.modes and *dyn* files are different. What is the difference between two vectors. And what is the unit of the vector in matdyn.modes?

######################################################################
 diagonalizing the dynamical matrix ...

 q =       0.0000      0.0000      0.0000
 **************************************************************************
     freq (    1) =      -0.000000 [THz] =      -0.000000 [cm-1]
 (  0.649741   0.000000     0.275541  -0.000000     0.043740  -0.000000   )
 (  0.649741   0.000000     0.275541  -0.000000     0.043740  -0.000000   )
     freq (    2) =       0.000000 [THz] =       0.000010 [cm-1]
 (  0.146859   0.000000    -0.243532   0.000000    -0.647399   0.000000   )
 (  0.146859   0.000000    -0.243532   0.000000    -0.647399   0.000000   )
#######################################################################

 3) When I use phonopy to calculate phonon dos. When I choose the same q denser in QE and Phonopy. But the output q is different. Do you know the reasons?
For example: In fcc Silicon, when the q density is set 2*2*2 in two software. The output Q is shown as below. 
#############
QE: Q POINTS
 q =       0.0000      0.0000      0.0000
 q =       0.5000     -0.5000      0.5000
 q =       0.0000     -1.0000      0.0000
phonopy:
- q-position: [    0.2500000,    0.2500000,    0.2500000 ]
- q-position: [   -0.2500000,    0.2500000,    0.2500000 ]
#############

 4)And what is the difference between phonopy and QE dfpt(Except the difference between finite displacement and dfpt)

Maybe I have so many questions. But I hope someone can help me!
Thanks so much in advance!

Best regards,
Zeli