[QE-users] Question on negative phonon frequency at Gamma point

[Yusuf Zuntu]

Dear all,

Please how do you get rid of a negative phonon frequency at a Gamma point. I have done some trial calculations for two different 2D materials. Please find the results details below:
System 1:
ph.x results at Gamma point
 q = (    0.000000000   0.000000000   0.000000000 ) 

 **************************************************************************
     freq (    1) =      -0.716980 [THz] =     -23.915891 [cm-1]
     freq (    2) =       1.170257 [THz] =      39.035571 [cm-1]
     freq (    3) =       2.301718 [THz] =      76.777040 [cm-1]
     freq (    4) =       4.692676 [THz] =     156.530836 [cm-1]
     freq (    5) =       6.547729 [THz] =     218.408714 [cm-1]
     freq (    6) =       7.575469 [THz] =     252.690440 [cm-1]
     freq (    7) =       7.954771 [THz] =     265.342590 [cm-1]

matdyn.x results
diagonalizing the dynamical matrix ...

 q =       0.0000      0.0000      0.0000
 **************************************************************************
     freq (    1) =      -0.168246 [THz] =      -5.612090 [cm-1]
 ( -0.205675   0.000000    -0.162406   0.000000    -0.049537   0.000000   )
 ( -0.204095   0.000000    -0.166786   0.000000    -0.047497   0.000000   )
 ( -0.202604   0.000000    -0.168584   0.000000    -0.038435   0.000000   )
 ( -0.204187   0.000000    -0.166901   0.000000    -0.037977   0.000000   )
 ( -0.204822   0.000000    -0.164879   0.000000    -0.035645   0.000000   )
 ( -0.204067   0.000000    -0.159939   0.000000    -0.043878   0.000000   )



System 2:
ph.x results at Gamma point
 q = (    0.000000000   0.000000000   0.000000000 ) 

 **************************************************************************
     omega( 1) =      -2.676536 [THz] =     -89.279630 [cm-1]
     omega( 2) =       2.176387 [THz] =      72.596456 [cm-1]
     omega( 3) =       2.268904 [THz] =      75.682505 [cm-1]
     omega( 4) =       2.781243 [THz] =      92.772280 [cm-1]
     omega( 5) =       2.831070 [THz] =      94.434329 [cm-1]
     omega( 6) =       5.362633 [THz] =     178.878184 [cm-1]
     omega( 7) =       5.391573 [THz] =     179.843530 [cm-1]

matdyn.x results 
diagonalizing the dynamical matrix ...

 q =       0.0000      0.0000      0.0000
 **************************************************************************
     omega( 1) =      -0.081634 [THz] =      -2.723002 [cm-1]
 ( -0.270214   0.000000    -0.101393   0.000000    -0.001203   0.000000   )
 ( -0.270014   0.000000    -0.100654   0.000000     0.001122   0.000000   )
 ( -0.270492   0.000000    -0.099822   0.000000     0.003548   0.000000   )
 ( -0.270726   0.000000    -0.100219   0.000000     0.003104   0.000000   )
 ( -0.271070   0.000000    -0.100621   0.000000     0.006056   0.000000   )
 ( -0.270797   0.000000    -0.101038   0.000000     0.002118   0.000000   )
 ( -0.270725   0.000000    -0.100220   0.000000    -0.002122   0.000000   )


Lastly, do you think the results above will affect Tc calculations. If Yes, please advice me on what to do to remove the negative frequencies in both cases.
Yusuf ZuntuPhD candidateSchool of PhysicsUSM, Maalysia




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