Re: [Phonopy-users] Imaginary frequencies in phonon frequencies

[孙亚宁 <sun...@gm...>]

Thank you for your reply.  I tried to use a 4*4*4 supercell, but the
calculation was too slow. The negative frequency has been solved for now.
Thanks again for your suggestion.

best wishes,
Yaning Sun


cammaant <cam...@fe...> 于2023年2月24日周五 15:24写道：

> Hello,
>
> I guess that, to plot the band structure, you are not using the same
> q-path as in the paper that you mention; however, if I'm correct, where you
> get the largest imaginary frequencies is the R point. Try to increase the
> supercell size along the diagonal, that is, use a 4x4x4 or 5x5x5, I suspect
> that the unit cell that you have is small and even a 3x3x3 is  not enough.
> The ISYM tag won't have any effect because the distorted supercells usually
> have symmetry P1.
>
> Best
>
> Antonio
> Il 24. 02. 23 7:03, 孙亚宁 ha scritto:
>
> Dear Prof. Togo,
>
> I have been calculating the phonon dispersion relation of Ag2O(space
> group:pn-3m) with phonopy package and there is always a imaginary
> frequency.  But in others' papers, no imaginary frequency appears in the
> phonon spectrum. I have tried many methods to eliminate the imaginary
> frequency, such as increasing the number of K-points, increasing the force
> convergence criterion for structural optimization, adding ISYM=-1, but none
> of them can eliminate the imaginary frequency.
> Here is the phonon dispersion relation I calculated with phonopy.
> [image: uTools_1677214049360.png]
> And this is the phonon dispersion relation for Ag2O from other paper.
> [image: uTools_1677215549212.png]
> This is the INCAR file that I use for phonon frequency calculation.
>
> SYSTEM = Ag2O
> PREC   = Accurate
> IBRION = -1
> NELMIN = 6
> ENCUT  = 500
> EDIFF  = 1E-8
> ISMEAR = 0
> SIGMA  = 0.01
> ALGO   = Normal
> ADDGRID= T
> LWAVE  = F
> LCHARG = F
>
> This is band.conf.
>
> ATOM_NAME = Ag O
> DIM = 2 2 2
> FORCE_SETS = READ
> BAND =0.0 0.0 0.0  0.0 0.5 0.0  0.5 0.5 0.0  0.0 0.0 0.0  0.5 0.5 0.5  0.0
> 0.5 0.0
>
> What should I do to deal with the imaginary frequency? Do you have any
> suggestions for me?
>
> Best wishes,
>
> Yaning Sun
>
>
>
> _______________________________________________
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> --
> _______________________________________________
> Antonio Cammarata, PhD in Physics
> Assistant Professor in Applied Physics
> Advanced Materials Group
> Department of Control Engineering - KN:G-204
> Faculty of Electrical Engineering
> Czech Technical University in Prague
> Karlovo Náměstí, 13
> 121 35, Prague 2, Czech Republic
> Phone: +420 224 35 5711
> Fax:   +420 224 91 8646
> ORCID: orcid.org/0000-0002-5691-0682
> ResercherID: A-4883-2014
>
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