[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.
  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.
  I will appreciate any answers.
  Thank you.
  Emine Kucukbenli

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