[Pw_forum] negative phonon frequencies for 1-D polymer chain
Ankit
ankitjain.me.iitk at gmail.com
Sun Apr 13 18:25:51 CEST 2014
Hello QE developers and users,
I am trying to calculate phonon dispersion using ph.x and matdyn.x
routines of QE and I am stuck woth negative frequencies for a while now.
I have a 1-D chain of polyethylene which has 6 atoms in the unit-cell.
The polymer chain is aligned in z-direction and I have added vacuum on
x- and y- direction. I did the structure relaxation and I am getting
total force after relaxtion as 0.000020, which I guess is sufficiently
small.
After relaxation, when I am trying to calculate phonon dispersion using
ph.x and matdyn.x I am getting netting frequencies for some of the
phonon modes. I searched online and on QE mailing list and tried couple
of different things:
1. The negative frequencies are appearing for non-gamma point as well,
so I guess its not a ASR issue. Anyway, I am using ASR but even after
this I am getting negative frequencies.
2. I reduced my tr2_ph and conv_thr to 1.0d-14 and 1.0d-15 which I guess
are very small as suggested on QE FAQs.
3. I checked phonon mode shapes for modes with negative frequencies and
they do not look like rotational mode.
4. I tried changing number of k points from 6 to 12 in z direction but
this is also not the cause.
I am really stuck here and not sure about the cause of negative
frequencies. With vc-relax, my structure is getting relaxed which I
guess means my structure is stable. But then I am not sure why I am
getting negative frequencies.
I really appreciate any help or suggestions.
Thanks,
Ankit Jain
IIT Indore,
Indore,
India
More information about the users
mailing list