[Pw_forum] R: Phonons with DFPT and DF2 functional

[Carlo Motta]

Hello Florian,

I tried to perform similar calculations some time ago, and found the same problems.
I am not 100% sure, but I think that with ph.x forces are computed without accounting for the dispersion interaction. This would mean that you are basically calculating phonons with a crystal structure that is not at the minimum energy relative to the xc potential you are using (since the relaxation is performed WITH vdW).

I hope that someone says that I am wrong!

Kind regards,
Carlo

--

Carlo Motta

PostDoc Researcher
School of Physics and CRANN,          IE Mob.: +353 (0) 83440 8930
Trinity College Dublin,                          IT Mob: +39 3286845055
Dublin 2, IRELAND
http://www.spincomp.eu/members/carlo-motta.xml
________________________________________
Da: pw_forum-bounces at pwscf.org [pw_forum-bounces at pwscf.org] per conto di Florian Altvater [altvater at berkeley.edu]
Inviato: mercoledì 5 novembre 2014 11.51
A: pw_forum at pwscf.org >> PWSCF Forum
Oggetto: [Pw_forum] Phonons with DFPT and DF2 functional

Hi,
I am interested in looking at effects of using van der Waals functionals
like vdW-DF2 on the phonon dispersion of organic crystals. Preliminary
results indicate that there might be some issues with using DFPT and DF2
together. More specifically, DFPT gives a couple "very imaginary"
frequencies (below -100cm^-1) for Gamma-point phonons (with and without
acoustic sum rule), while finite displacement results seem to be just
fine, using the same convergence parameters.

So before I dig deeper into eventual other issues at play here, I wanted
to ask if you know of any incompatibilities when using DFPT and vdW-DF2?
Either inherently embedded in the underlying theory (which I could then
dig into first), or just in the way it is implemented in QE.

Thanks for your help,
Florian

---
Florian Altvater
PhD candidate
UC Berkeley/LBNL
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