[Pw_forum] negative phonon frequencies for 1-D polymer chain

[Ankit]

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