[Pw_forum] negative phonon frequency in GaN bulk

[Paolo Giannozzi]

On Apr 10, 2010, at 15:47 , mohnish pandey wrote:

> As you can see there is perssure is different in final vc-relax and  
> scf calculation.

this has been explained no less than 36000 times: the number of plane  
waves
depends upon the starting cell volume of a vc-relax calculation, so  
it may differ
from the number of plane waves used in a scf calculation with the  
same final
geometry. In your case the difference in presure is so ridicolously  
small that
it doesn't deserve to be called a difference.

> After that I am doing phonon calculation and I am getting some  
> frequencies to be negative.

and this has been explained a litle bit more than 72000 times:
http://www.quantum-espresso.org/user_guide/ 
node79.html#SECTION00097030000000000000

P.
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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