[Pw_forum] phonon calc. for fcc at X

[Stefano Baroni]

Emine: (001) and (011) are *NOT* two "X" points in the FCC Brillouin  
zone. The difference between the second and the first (010) is *NOT*  
a reciprocal-space vector. If it was, X would also be, which you will  
agree it is not. As a matter of fact, (011) has its own nickname (it  
is called "K"). No surprise, therefore, that frequencies @K are  
different from frequencies @X. Are these arguments clear and  
convincing enough? Stefano


On Dec 9, 2006, at 11:10 PM, e kb wrote:

> Dear QE users,
> I am performing phonon calculations for a zinc blende structure,
> I calculated the frequencies at high symmetry points, evaluating a  
> single point at each time.
> So I followed the procedure of scf, nscf and ph.x calculation one  
> by one for L(0.5 0.5 0.5) - Gamma - X(0,0,1) - X(0,1,1) points.
> In these calculations every parameter (else than q-vector in  
> &phonon list in nscf calculation and phonon card for ph.x input of  
> course) is kept fixed.
>
> Although I expect the frequencies at two X points - (0,0,1) and  
> (0,1,1) - to be the same,
> the difference in TA modes' frequencies are around 30 cm-1,
> in the other ones, around  5 cm-1.
>
> I would like to attribute this to unconverged lattice parameters,  
> cut off energies or thresholds etc. but what I do not understand is  
> why the program treats these two points differently, although they  
> should be equivalent. So I expect to see the same value for  
> frequency at these points, even if it is wrong, highly negative, or  
> sth else that it should not be.
>
> I will appreciate any answers.
> Thank you.
>
> Emine Kucukbenli
>
>
>
> Everyone is raving about the all-new Yahoo! Mail beta.

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
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