[Pw_forum] 回复： Re: [Pw_forum] negtive values for TA when using q2r.x

[Katalin Gaal-Nagy]

Dear Li Yan,

I was checking my very old work today (it was my diploma work, I am 
sorry, it is not published and it is in German), and the final solution of 
my former problem with my numerical instability was an increase of the 
k-points of the Ground state together with an increase of the number of 
q-points for the phonons. On Monday I can send you some figures.
It was not enough just to increase the number of q-points.

Have a nice weekend,
best wishes,
Katalin


>      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 
> A values.
>
>  thank you for your advice.
>    best regards
>     Li Yan
>
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