[Pw_forum] phonon calc. for fcc at X

e kb eminekb at yahoo.com
Sat Dec 9 23:10:36 CET 2006

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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