[Q-e-developers] Fwd: negative phonon frequencies problem

[Paolo Giannozzi]

Begin forwarded message:

> From: Shouting Huang <huangshouting at gmail.com>
> Date: June 15, 2010 3:44:36  GMT+02:00
> To: giannozz at democritos.it
> Cc: paolo.giannozzi at uniud.it
> Subject: negative phonon frequencies problem
>
> Dear Paolo Giannozzi,
>
> I am trying to calculate phonon dispersion of graphene. The result  
> is good without external electric field. But when I add an external  
> electric field (sawtooth potential), some acoustic modes phonon  
> frequencies near Gamma point become negative. Even I reduce the  
> electric field to an extremely small value of E=0.001 V/nm, the  
> negative frequencies are still obvious, almost indifferent from the  
> case when E=10 V/nm. You can see the dispersion curves in the  
> attached files.
>
> Some reasons might explain the causes of negative frequencies:
>
> 1. instability of the chosen structure.
> But I relax the structure, the difference between the initial input  
> structure and the final structure is tinny.
>
> 2. too large convergence thresholds:
> I use ecutwfc =60.0, conv_thr =  1.0d-10, tr2_ph=1.0d-18. I don't  
> think that these thresholds are large.
> k-points in ph.x are 8 X 8 X 1. And I do check the running result,  
> convergences are achieved every step.
>
> So I really don't understand the cause of negative frequencies in  
> my calculation. Why there is large difference between the case with  
> electric field and the case without electric field, but there is  
> little difference between large electric field case and small  
> electric field case?
>
> Could you give me some suggestions on this problem? Thanks very much.
>
> Shouting Huang
> Department of Physics,
> Washington University in St. Louis
> Saint Louis, Missouri, USA 63130
> Email: shuang at physics.wustl.edu
>
>
>
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Paolo Giannozzi, Dept of Physics, University of Udine
via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222