[Pw_forum] phonon dispersion calculations

[Eyvaz Isaev]

Hi,

You can generate these q-points for yourself. 
The number of q-points depends on how the phonon
spectrum is complex (how many intersections, softening
of phonon branches). 20 for each high symmetry
direction looks reasonable.

Just have a look Bradley-Cracknell textbook about BZ
vertices and then divide each direction as much as you
like (see above). Do not forget that you have to
transform these q-points to cartesian. 

Bests,
Eyvaz.


--- Tuhina Kelkar <tr.kelkar at ncl.res.in> wrote:

> Hi! I am a new user of QEspresso. I want to use it
> to calculate the phonon dispersion for MgH2 along
> the following high symmetry points: Gamma to X to M
> to Gamma to Z to R to A to Z.
> But I am not being able to construct the matdyn.in
> file. How does one decide on the number of q-points?
> And how to go about creating this list of q-points
> to be used in matdyn.in?
> I have already obtained the IFC's and the following
> is the resulting q2r.out file:
>   reading grid info from file dyn0
>   reading force constants from file dyn1
>   Dielectric Tensor not found
>   nqs=            1
>  q=   0.00000000  0.00000000  0.00000000
>   reading force constants from file dyn2
>   nqs=            1
>  q=   0.00000000  0.00000000 -0.74850299
>   reading force constants from file dyn3
>   nqs=            2
>  q=   0.00000000 -0.50000000  0.00000000
>  q=   0.50000000  0.00000000  0.00000000
>   reading force constants from file dyn4
>   nqs=            2
>  q=   0.00000000 -0.50000000 -0.74850299
>  q=   0.50000000  0.00000000 -0.74850299
>   reading force constants from file dyn5
>   nqs=            1
>  q=  -0.50000000 -0.50000000  0.00000000
>   reading force constants from file dyn6
>   nqs=            1
>  q=  -0.50000000 -0.50000000 -0.74850299
>       q-space grid ok, #points =    8
>       fft-check success (sum of imaginary terms <
> 10^-12)
> Thanks in advance.
> Tuhina
> 


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