[Pw_forum] phonon calc. for fcc at X
baroni at sissa.it
Sun Dec 10 16:07:11 CET 2006
Dear Emin, dear all: all my apologies!
Nicola was of course right, and I was WRONG! (011) IS indeed
equivalent to (001), because the latter is equivalent to (-1,0,0) and
the difference between (011) and (-100) is (111) which IS an FCC
reciprocal-lattice vector. The coordinates of the K point are
(3/4,3/4,0). With reference to the figure mentioned in the link
below, K is the intersection among the planes x+y+z=3/2, z=0, and
x=y, hence the above coordinates.
To come to your original question. Are the symmetries/degeneracies of
the results what you expect? Can you make the input/output of your
calculation accessible to us?
On Dec 10, 2006, at 2:46 PM, e kb wrote:
> 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
>> Everyone is raving about the all-new Yahoo! Mail beta.
> Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center -
> [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
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Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center -
[+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
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