[QE-users] Doubt regarding the phonon calculations of RbPbI3

[Eduardo Menendez]

Hi Anupriya,
If decreasing the convergence threshold does not solve or modify
significantly the imaginary frequencies, the problem may be due to the fact
that atoms are in high symmetry positions. See that

ATOMIC_POSITIONS {crystal}
Rb       0.250000000   0.584491575   0.824889298
Rb       0.250000000   0.084378763   0.675213228
...
the a-coordinates are clearly of high symmetry. The b- and c-coordinates
are not, apparently, but may be due to celldm(2)=2.185 anf  celldm(3)=3.657

Perovskites often suffer distortions with respect to the ideal positions,
and the negative (imaginary) frequencies may reflect an incomplete
relaxation.

I suggest shaking the structure a bit, either applying random displacements
to the coordinates, or running a short molecular dynamics. Afterwards,
relax the system using  the option nosym=.true.

Variable cell relaxation may also help. In principle, a stressed crystal
should also have phonons with real frequencies, but the stress could induce
a phase transition to a structure with a larger unit cell.

Good luck!

Eduardo Menendez Proupin
--------------------------------------


>
>
> ---------- Forwarded message ----------
> From: Anupriya Nyayban <mamaniphy at gmail.com>
> To: users at lists.quantum-espresso.org
> Cc:
> Bcc:
> Date: Wed, 22 Sep 2021 19:13:27 +0530
> Subject: [QE-users] Doubt regarding the phonon calculations of RbPbI3
> I have calculated the phonon dispersion of orthorhombic RbPbI3 (Pnma)
> using "PHONON" as implemented in Quantum Espresso and found the negative
> frequencies ( which should not be present as suggested in
> https://doi.org/10.1063/1.5131575). It would be great help if you suggest
> to me where I am doing wrong!!! I have provided the computational details
> below:
> 1) The structure first is being relaxed and then the convergence tests are
> performed for Ecut, Kmesh and Lattice parameters.
> 2) Using the relaxed structure with converged parameters, SCF is performed
> with cov_thr=1.0d-8 (ecutwfc=70, ecutrho=600, kmesh=12*6*3); phonon
> calculation is performed with tr2_ph=1.0d-14 at Gamma point, frequencies
> are obtained by imposing acoustic sum rule at the Gamma point with
> asr=simple.
>
> Thank you!!
>
>
> --
> With regards
> Anupriya Nyayban
> Ph.D. Scholar
> Department of Physics
> NIT Silchar
>
>
>
>
>
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