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

Kyeong-hyun Park kpark39 at gmail.com
Fri Oct 12 01:14:29 CEST 2012

Dear Dr. Lorenzo Paulatto

Hi, this is Kyeong. I was having hard time to figure out how to post
questions on the PW_forum and tried different ways from different
accounts. That's why my posting was from anonymous name. I also
thought it might be easier to explain my problems with the simple

Here's the real problem. I am generating phonon dispersion of bismuth
telluride which has a rhombohedral structure with 5-point basis, and
it generates 15 different frequency modes. As I explained in the
previous message, the optical frequencys near gamma point are
different depending on the propagation directions inside the FBZ. When
I only calculated the frequencies at Gamma, it seemed okay, but it
went weird as I added even one more q-vector to be calculated.

I looked into my input files. I set the structure system as 'ibrav=5'
(trigonal) because i found out that it's primitive cell is a simple
rhombohedron. I set the K_POINTS as 'automatic 4 4 4 0 0 0' (This may
be the one that I need to change to get symmetric structure), but I am
not sure how to define the k points. To manage LO-TO splitting, I also
set 'epsil=true'. That's pretty much what I have in my input files.

I hope this is enough description to get a meaningful help. Any advice
will be appreciated. Thank you.

Best Regards,

>* Hi,*>**
Dear dfsgdv dfsdf,

>* I will explain my problmes with a simpe example.*>**>* Why don't you explain posting some useful details of the actual*calculation you have done?

Assume I am doing the phonon dispersion [ ...]
>* So basically my problem is that I have generated two phonon dispersion*>* relation: Gamma to Z and Gamma to X, but the optical mode frequencies at*>* Gamma in the two plots are different.*>**
Either there is an error in your input, or the phonon code is smarter than
you. I.E. it is applying direction-dependent LO-TO splitting to the modes
at Gamma (one ref: <http://cmt.dur.ac.uk/sjc/thesis_prt/node62.html>)

If there is indeed a problem with your calculation, you should give us more
information in order to get meaningful help.

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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