[Pw_forum] phonon calc. for fcc at X
Stefano Baroni
baroni at sissa.it
Sun Dec 10 10:34:59 CET 2006
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
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
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