[Pw_forum] Three questions about phonon calculation

Prasenjit Ghosh prasenjit.jnc at gmail.com
Wed Jun 1 16:10:51 CEST 2011

>  Dear all,

Dear  Yu Han Ho

> I have some questions about Phonon calculation.
> 1. If I use ph.x to run the graphene phonon dispersion and pdos,
>     should I use the q points which is multiple of 6 because of the
> hexagonal shape of grathpene crystal?

It need not be multiples of 6.....what you should keep in mind is that you
should odd q-point unshifted grid (i.e. 3x3 or 5x5 or 6x6) so that you
retain the symmetry of the unit cell.

> 2. When I run phonon-dispersion calculation, it always cost very long cpu
> time. Is that normal?

It is expensive......for each mode the calculation time is of the order of
one scf cycle.

> 3. Can QE calculate vibration mode?
>     How could I set terms in ph.in to get vibration mode?

The eigenvectors corresponding to each eigen value represent the vibrational

In order to find out how to set up the input files please refer to the

>     And how could I plot the vibration mode from the output file?
I do not know whether there is some direct post processing utility available
to do that.....however, you can use xcrysden to plot the modes......in order
to know how to do it you can refer to the following link:


In place of Fx, Fy and Fz, you need to put in the eigen vectors.

> Thanks a lot.
You are welcome. Also please remember to provide your affiliation.


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