[Pw_forum] »Ø¸´£º Re: [Pw_forum] negtive values for TA when using q2r.x

[Eyvaz Isaev]

Hi, 

If I understood correctly, you have got imaginary (but
not negative!, as you mentioned) frequences for X(100)
point using 4x4x4 q-mesh. If so, the use of 6x6x6 mesh
which also has (100) point is hopeless. 
Appearance of imaginary frequency at (100) point is a
usual case for unstable compounds within NaCl
structure.

I suggest, it is somewhat strange to do FFT (q2r.x)
having an imaginary frequency and then expect a stable
structure. These imaginary frequences near Gamma point
you found using q2r.x,  are, presumably, connected to
X(100) point instability.

So, I guess, the problem might be due to:
a) low cutoff energy
b) insufficent k-points mesh
c) quality of a pseudopotential 
d) check carefully your input file 

Best regards,
Eyvaz.


--- li yan <liyanpcl at yahoo.com.cn> wrote:

>       Dear katalin,
>       
>       I want to find the reason of the phase
> transiton for AgF by calculating the phonon
> dispersion curves. My intent is finding the soft 
> mode at high pressure.
>     
>   >Which was the first grid, which you used to
> calculate the dispersion?
> 
>       AgF is fcc strcture, c/a is 1, so the mesh was
> initially chosen as 4*4*4. 
>    
>   At 3GPa, after calculating the eight special
> points, i found only one of them (0,0,1) has a
> negative mode.   Then  i did the calculation  of the
> phonon disperion curves with q2r.x, the frequences
> of TA near the gama point  are negtive.  Near gama
> point, i just selected one q, whose TA is negtive, 
> to calculate the phonon, but the value of the TA is
> positive. I thought  the q mesh is  small, so i
> chose 6*6*6 mesh. 
>    
>   > Does the 6x6x6 mesh include also the points you
> calculated for checking. 
> 
>       The 6*6*6 mesh dosen't include the above 
> modes which has negtive values, but i cannot find
> other ways to avoid the unexpected negtive values. 
>    
>   thank you for your advice.
>     best regards
>      Li Yan
>   
>  
> 
> 
> 
> 
> 		
> ---------------------------------
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