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

Lorenzo Paulatto lorenzo.paulatto at impmc.upmc.fr
Tue Oct 16 10:14:25 CEST 2012

On 16 October 2012 05:09, Kyeong-hyun Park <kpark39 at illinois.edu> wrote:

> Dear Dr. Lorenzo Paulatto
> Thank you for your suggestion though, unfortunately commenting out the
> statement didn't resolve
> my problem.

Well, I think your first problem was solved. However I'm not sure if you
 understand why there was an anisotropy in your frequencies, and the fact
that the anisotropy was good, i.e. a real effect of your system.

> I thought I was pretty close, but now it seems that it's one of the input
> variables.
> Based on what did you realize that my matdyn complained for non-converged
> or non-stable system?
> Would lowering either convergence threshold (conv_thr) or threshold for
> selfconsistency (th2_ph) make
> my system more stable? For now I have 'conv_thr=1.0d-8' and
> 'th2_ph=1.0d-14'

I see that you use 5 inequivalent atomic type to model just 2 types of
atoms, this means you are losing some symmetry. Symmetry helps a lot with
phonons, both in terms of stability and computing time. Do you have any
specific reason to do so?

I would say you probably have to increase the number of kpoint, and/or the
wavefunction and/or charge density cutoff. When you are studying the phonon
dispersion it is often necessary to test convergence of the phonon
dispersion, i.e. compute at different values of cutoff and verify that it
does not change much.

If you have someone who is more experienced with DFT calculations in your
institution I suggest you contact him/her as it is difficult to understand
exactly what's your problem via email.

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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