# [Pw_forum] phonon dispersion calculations

Eyvaz Isaev eyvaz_isaev at yahoo.com
Tue Aug 8 16:02:49 CEST 2006

```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.

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
>   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)