# [Pw_forum] Different optical mode frequency at the same q-vector

Lorenzo Paulatto lorenzo.paulatto at impmc.upmc.fr
Mon Oct 15 09:23:07 CEST 2012

On 15 October 2012 07:24, Kyeong-hyun Park <kpark39 at illinois.edu> wrote:

> Meanwhile, I will try different 'K_POINTS' setup in 'scf.in' that have
> been set as 'automatic 4 4 4 0 0 0'
> I thought '4 4 4 0 0 0' would be okay for most of structures even though
> it will take longer time due to not pre-defined symmetry points.
> I will give it a shot with '6 6 6 0 0 0'. Please let me know if you have
> any better suggestion.
> For the structure like rhombohedron, should it be some thing like '4 4 6 0
> 0 0' ?
>

Dear  Kyeong-hyun Park,

1. K points have nothing to do with your problem. Phonon q-points and
electronic k-points have nothing to do with each other except for having
the same unit of measure (unless you are doing electron-phonon interactions)

2. phonon frequency at the Gamma point are computed by matdyn with a
formula that is 100% direction independent:
D(q=0) =  \sum_R F_{0,R}
even if you put random data in the force-constants file the result will
NOT depend on the direction.

3. the only direction dependent term computed by matdyn is the LO-TO
splitting, you think you have disable it in the code, but maybe you have
not (the code is unnecessarily complicated)

4. I do not have a handy example to check out how to enable/disable LO-TO
splitting, but if you want to send me the *dyn files I'll have a look

bests

--
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 44275 084 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
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