[Pw_forum] Wrong degeneracy

Stefano Baroni baroni at sissa.it
Thu Mar 3 14:32:10 CET 2005

```On Mar 3, 2005, at 1:14 PM, Shihn Lun wrote:

> Hi Gironcoli,
>
> And I dont understand why the N point is (0.5 0.5 0.0)
> in pwscf? Could you give me some hint?

A basis for the reciprocal lattice of the BCC lattice is, e.g., {(110),
(101), (011)}, in units of two pi over a. The N point is not
(1/2,1/2,0) "in PWSCF", but it is so "in cartesian coordinates" (which
are in fact used by PWSCF/PHONON to represent vectors in the Brilouin
zone). The rationale is:

N = (1/2,0,0) [in the base {G1,G2,G3}] = 1/2*G1 + 0*G2 + 0*G3 = 1/2
(110) = (1/2,1/2,0) [in Cartesian coordinates].

Hope this clarifies the matter.

Stefano B

>
>
>
> Shihn Lun
> --
>
> --- Stefano de Gironcoli <degironc at sissa.it> wrote:
>
>> Shihn Lun wrote:
>>
>>> Hello Baroni,
>>>
>>> I want to check my phonon calculation with
>>
>> PRL.v77.p1151.(http://prola.aps.org/abstract/PRL/v77/i6/p1151_1)
>>>
>>>
>>> Shihn Lun
>>>
>>>
>> If you read carefully the figure caption of PRL 77,
>> 1151 (1996) it is
>> said that
>> "directions of propagation and polarizations are
>> espressed in terms of the
>> reciprocal lattice vectors"
>>
>> The N point in a BCC Bravais lattice ---
>> corresponding to (0.5,0.0, 0.0)
>> in that figure ---
>> IS NOT the (0.0 0.0 0.5)  point (in cartesian
>> coordinates in unit of
>> 2pi/alat) you are
>> computing with ph.x !
>> It would rather be (0.5,0.5,0.0)
>>
>> best regards,
>>
>> stefano de Gironcoli
>>
>> _______________________________________________
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>>
>
>
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---
Stefano Baroni    ---  SISSA  &  DEMOCRITOS National Simulation Center
via Beirut 2-4 34014 Trieste Grignano / [+39] 040 3787 406 (tel) -528
(fax)

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