Re: [Phonopy-users] Negative frequencies for FCC Al

[Roginskii E. <rog...@ho...>]

Hi,
This is most probably calculator issue. I don't know how EMT acts, but maybe there are some additional parameters needed like cutoff energy or k-points mesh. Moreover sometimes it is worth to apply acoustic sum rule (FC_SYMMETRY option of PHONOPY).

Eugene.

On 07/27/2017 04:59 AM, eric hoglund wrote:
Dear users,

I have been attempting to model the phonon dispersion of FCC aluminum, which should be a fairly simple task. I would like to use phonopy instead of ase's phonon module due to some additional features. When plotting, all of my acoustic bands appear negative, any ideas why? I have read online in different locations that this is due to instability or poor potentials.  But running this in other software produces positive frequencies (and acoustic should be stable for relaxed fcc aluminum). I have followed a process similar to the Si-8.py example, but I have removed the 0.25&0.75 basis to make it FCC instead of diamond. In addition I am using the EMT calculator supplied by ASE. My file is attached.

Other short questions:
-when I use a supercell>7x7x7 the system seems to lock up on creating the primitive vectors. If it passed that it will lock up on getting the force constant. Is this just an efficiency thing? ASE's phonon portion doesn't seem to have the same issue.

Thank you,
Eric Hoglund
University of Virginia
PhD student
1(919)538-1536<tel:%28919%29%20538-1536>



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