# [Pw_forum] phonon calc. for fcc at X

e kb eminekb at yahoo.com
Sun Dec 10 14:46:36 CET 2006

```Thank you for your answer Stefano,
Of course this explains the different frequencies,
and that also means that I misunderstand the kpoints given in example 5,
about band structure for an fcc,
and Dr. Marzari's explanation to another QE user,
who was confused like me.
http://www.democritos.it/pipermail/pw_forum/2006-June/004365.html

Thank you again,
Emine Kucukbenli

Stefano Baroni <baroni at sissa.it> wrote:
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.

Thank you.

Emine Kucukbenli

---------------------------------
Everyone is raving about the all-new Yahoo! Mail beta.

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

Please, if possible, don't  send me MS Word or PowerPoint attachments
Why? See:  http://www.gnu.org/philosophy/no-word-attachments.html

---------------------------------
Check out the all-new Yahoo! Mail beta - Fire up a more powerful email and get things done faster.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20061210/6f5e3d22/attachment.html>
```