[Pw_forum] phonon calc. for fcc at X

[e kb]

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

  
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  Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
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