[Pw_forum] About getting Raman Spectrum for a 24 atom-hybrid halide perovskite

[Josue Itsman Clavijo Penagos]

Dear all, I apologize for sending a former e-mail from another account
other than the subscribed one.

So, here is my post again:

I’m having trouble to replicate the result for the raman spectra for some
hybrid perovskite, namely CH3NH3PbI3, a.k.a MAPbI3 (4 MAPbI3 molecules/unit
cell). I'm using the relaxed cell and atomic coordinates as in J. Phys.
Chem. Lett., 2014, 5 (2), pp 279–284.

So, I’d really thank you any advice and words of correction regarding to
the following issues:

1-The thing is that my phonon calculations never converge, even using an
energy cutoff in the scf previous calculations of 1D-12. (Using
Norm-conserving LDA-level PZ PPs). WIll be necessary to lower the tr2_ph
value in ph.x to something greater than 1D-12 since the system is somewhat
huge (24 atoms)?

2- In a latter ph.x run attemp, I set the tr2_ph value to 1D-8 and the the
initial convergence was achieved, but at the stage of calculation the
second derivatives, roots never converged, so i can not get the dynamical
matrix anytime.

So, for as a huge system as MAPbI3 is, It is likely to exist special
constraints, adjustments or requirements i'm missing, in order to get the
dynamical matrix and, using dynmat.x and other plotting tools, to simulate
the raman spectrum with the quantum espresso suite?

Excuse me if i’m missing several important theoretical or QE-related
aspects, but i really need some advice about all of this stuff.

Best regards,
-------------------------------------------------------
*Dr.Sc. JOSUÉ CLAVIJO, Chem.*
*Christian, born again*

Assistant Professor
Universidad Nacional de Colombia
Science College
Chemistry Department
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