[Pw_forum] the symmetry problem in calculating phonon frequency of wurtzite AlN

[Ari P Seitsonen]

Dear ??, [ Please notice the humble rules how to post to the list: 
Affiliation ]

   I am not the best expert here, but I presume that you refer to the space 
group C_6v, but QE/pw.x only finds the point group C_3v. I am not sure 
whether the phonon code could handle the translations (probably not), but 
I do not think that this should lead to any serious differences between 
the two symmetry groups (in particular since you are using a 
more-than-sufficient 'ecutwfc'; well, I do not know whether it would be 
better to reduce that and use a value for 'ecutrho' of typical 6-8 times 
'ecutwfc'). How large is your deviation from the "reference" (what do you 
use as reference?)? Did you consider the LO-TO splitting? You have quite 
few k points - and once again, using the lateral shift of k points in a 
hexagonal cell "spoils" the symmetries - sorry for repeating this, but it 
does not seems to be clear to the wide community.

     Greetings from Sunny Paris,

        apsi

-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
   Ari Paavo Seitsonen / Ari.P.Seitsonen at iki.fi / http://www.iki.fi/~apsi/
     Ecole Normale Supérieure (ENS), Département de Chimie, Paris
     Mobile (F) : +33 789 37 24 25    (CH) : +41 79 71 90 935


On Tue, 12 Apr 2016, wz wrote:

> Dear all,
> 
> I am trying to calculate the phonon frequency of wurtzite AlN in gamma
> point?? i try to use the input files as PHonon/examples/example01/ in
> Quantum Espresso. The symmetry of wurtzite AlN is C6v, but in the output of
> my calculation, the symmetry is C3v so that the phonon frequency get by my
> work do not correspond with that get by reference.  
> I want to know that if there is a way in quantum espresso to specify the
> symmetry? or some other method to get the right symmetry in phonon
> calculation?
> ===================================
> pw input
>  &SYSTEM
>     ibrav= 4, celldm(1)= 5.8804, nat= 4, ntyp= 2,
>     celldm(3) = 1.600724
>     ecutwfc = 160
>  /
>  &ELECTRONS
>     conv_thr =  1.0d-8
>     mixing_beta = 0.7
>  /
> ATOMIC_SPECIES
>     Al   26.982  Al.pbesol-n-kjpaw_psl.0.1.UPF
>     N    14.007  N.pbesol-n-kjpaw_psl.0.1.UPF
> ATOMIC_POSITIONS crystal
>     Al  0.333333343         0.666666687         0.000000000
>     Al  0.666666627         0.333333313         0.500000000
>     N   0.333333343         0.666666687         0.381929994
>     N   0.666666627         0.333333313         0.881929994
>        
> K_POINTS automatic
> 2 2 1 1 1 1  
> ------
> ph input
> 
> phonons of aln at Gamma
>  &inputph
>   tr2_ph=1.0d-14,
>   prefix='aln',
>   epsil=.true.,
>    outdir='/tmp',
>    fildyn='aln.dyn',
>  /
> 0.0 0.0 0.0
> 
>