[Pw_forum] Frequency vibration at gamma point from dynmat.x and q2r.x + matdyn.x

[loc duong ding]

Dear all,
I am trying to calculate phonon dispersion. Firstly, I try to run phonon calculation at gamma point and use dynmat.x to get the vibration frequency at gamma. It imposed well frequencies with zero for 3 acoustic modes.  

After that, I try to calculation phonon using grid and calculate force by q2r.x. The phonon dispersion was calculated by matdyn.x. However, I got a negative value of acoustic modes at gamma point. The value is around -20cm-1. 

I have used asr=simple for all calculations. 

Could you please give me some hints to overcome this issue? It seems negative value originated from numerical error (dynmat.x imposed well zero frequency) but I can not understand why they imposed negative values by matdyn.x. 

I would like to thank for all your instructions.  Best regards, 
-----------------------------------------------
Dinh Loc Duong , Ph.D
Postdoctoral Researcher, Department of Nanoscience
Max Planck Institute for Solid State Research
Heisenbergstrasse 1, D-70569 Stuttgart, Germany             
Email: mambom1902 at yahoo.com, l.duong at fkf.mpg.de 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20141227/5c536f2c/attachment.html>