[Pw_forum] What is the unit of q point in ph.x output file?

[Lorenzo Paulatto]

On Sat, 15 May 2010 16:21:47 +0200, xirainbow <nkxirainbow at gmail.com>  
wrote:
> *Question one: *How to write the input file of ph.x to get phonon  
> dispersion along a special line.

There is a set of scripts from Eyvaz Isaev that does that, I think. They  
should be included in the most recent version of QE. However, the standard  
way to do it is compute phonons on a regular grid and then use Fourier  
interpolation to have a nice smooth curve along the path of your choice.  
See example06 for details.

> *Question two:* What is the unit of q point?

It is 2 pi/alat, where alat is the lattice parameter (i.e. what you  
specified as celldm(1), or as "a" in the pw.x calcualtion)

> *      Dynamical matrices for ( 4, 4, 4,) uniform grid of q-points*
> *      with only ( 4, 2, 2,) point requested*
> *      (   1q-points):*
> *      N       xq(1)       xq(2)       xq(3) *
> *      1     0.50000     0.50000     0.50000 "*
> *      Calculation of q =   0.5000  0.5000  0.5000*
> *Question three:* Why                    *q=   0.5  0.5  0.5?*
>                       I think it should be *q= -0.5  0.5  0.8713185.*

I think you are right, well actually it would be 0.5 -0.5  0.8713185.
But I've noticed that the subroutine computing the value of the q-point is  
actually called *after* the one the prints it; I guess the calculation is  
probably done at the correct point, it is the output on screen that is  
wrong. At the end of the calculation you can check inside the dynamical  
matrix file (matdyn) to be sure.

> *Question four: Is unit of q point for "ldisp = .true." and "ldisp =
> .false." the same? *

Yes, I think it is just the problem is the one I was stating above.

> *Thanks in advance：）*


you're welcome, thank you for reporting

-- 
Lorenzo Paulatto

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