[Pw_forum] including vdwl interactions during phonon calculations

[Vikas Varshney]

Dear QE users,
I tried looking for answer for this question (the one in the subject) but
could not find, hence I am asking this.

I wanted to calculate phonon dispersion curves for layered materials like
MoS2. I have successfully relaxed the crystal structure of both of these
systems using vc-relax and including vdwl interactions (using .london. flag)
and got all the relaxed crystal dimensions within 1.5% of experimental
values.

Now I want to run ph.x to calculate normal modes for different wave vectors.
However, I could not find, how I can include vdwl interactions in these
calculations? It is also interesting to note that in ph.x input script,
there is no need to specify pseudopotential file information. So, I am
suspecting it is gathering information from certain directories which were
created during scf run.

In order to test whether vdwl interactions are taken into account in phonon
calculations implicitly, I ran two phonon cases (in two different
directories) after successful convergence of two SCF calculations (one
including .london option and one without including that option). The
coordinates and cell dimensions for both calculations were identical. The
initial structure was based on vc-relaxed structure with .london=true was
from previous vc-relax calculations.

On looking the predicted frequencies, I find that both cases yeilded exactly
same numbers for frequencies. This is pointing me to believe that while
calculating dynamical matrix, vdwl interactions are not taken into account
implicitly. Am I wrong? Is there a way to include such interactions?

In addition, I got one of the 12 predicted frequencies to be -ve (~45 cm-1).
I am not sure if it is because of non-inclusion of vdwl interactions. I also
wanted to mention that higher optical modes are predicted very near to
experimental values (within 10 cm-1) for all IR and Raman active modes.   I
have read many such questions before on the forum on this issue (-ve
frequency) and would be trying different suggestions but I wanted to address
the issue of inclusion/exclusion of vdwl interactions in phonon calculations
and how one can you about calculating their contribution towards dynamical
matrix.

Best Wishes,

Regards,
Vikas Varshney
Computational Materials Scientist
Wright Patterson Air Force Base, Dayton, OH



Best Regards,

Vikas Varshney
Computational Research Scientist,
Wright Patterson Air Force Base, Dayton OH
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