[Pw_forum] Phonons with DFPT and DF2 functional

[Florian Altvater]

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

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Florian Altvater
PhD candidate
UC Berkeley/LBNL