[Pw_forum] dismatch of Gamma phonon frequencies from different way

[黄志硕]

Dear all,
I was calculated Gamma point phonon of layered HfS2 through single point calculation, the results is 
     freq (    1) =0.00 [cm-1]
     freq (    2) =0.00 [cm-1]
     freq (    3) =0.00 [cm-1]
     freq (    4) =176.91 [cm-1]
     freq (    5) =176.91 [cm-1]
     freq (    6) =269.40 [cm-1]
     freq (    7) =269.40 [cm-1]
     freq (    8) =338.76 [cm-1]
     freq (    9) =340.88 [cm-1]
which is consistent with Xiaokun Gu's work which had been arranged in the paper of http://arxiv.org/abs/1407.3758.


But when I calculated the phonon-dispersion based on a q-mesh of 4*4*1, after q2r.x and matdyn.x calculation, the Gamma point phonon frequency in the "matdyn.modes" file differed a lot:
     freq (    1) = -0.000007 [cm-1]
     freq (    2) =-0.000006 [cm-1]
     freq (    3) = -0.000001 [cm-1]
     freq (    4) = 176.907415 [cm-1]
     freq (    5) =269.403108 [cm-1]
     freq (    6) =269.403108 [cm-1]
     freq (    7) = 298.948294 [cm-1]
     freq (    8) =338.760427 [cm-1]
     freq (    9) =340.877820 [cm-1].
The parameters in the input file are: ecutwfc = 90, ecutrho = 900, conv_thr =  1.0d-8, mixing_beta = 0.7, etot_conv_thr and forc_conv_thr are set default, and the K-mesh is set 17 17 2 1 1 1. During phonon calculation, "ASR" is set 'simple'.


Before calculation, crystal structure was fully relaxed, and the convergence of K-mesh and cut-off of plane wave was achieved.
I also calculated the phonon-dispersion based on q mesh 4*4*2 and 8*8*1, and got the same results with that of q mesh 4*4*1.


I have no idea why the phonon frequencies of Gamma point get from single-point calculation and phonon-dispersion calculation differ so much. I will appreciate it if anyone could give me some instructions.

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