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

[Tenney, Craig Michael]

Hello Yusuf,


A negative phonon frequency can be a sign that your material is dynamically unstable to vibrations.


This does not pose a problem in calculating Tc values.  You can include the negative frequencies and Tc will be slightly reduced, or you can manually cut them out if you think it makes more sense in your case.


Hope this helps, if you have any more questions, feel free to ask,


Craig Tenney

Washington State University

Graduate Student

________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of Yusuf Zuntu <yzunt at yahoo.com>
Sent: Thursday, March 22, 2018 9:41:23 PM
To: users at lists.quantum-espresso.org
Subject: [QE-users] Question on negative phonon frequency at Gamma point

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 Zuntu
PhD candidate
School of Physics
USM, Maalysia




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