[Pw_forum] including vdwl interactions during phonon calculations

Vikas Varshney vv0210 at gmail.com
Tue Jun 28 17:25:29 CEST 2011

Dear Stefano,
Thank you very much for the email. I would need some guidance
regarding how to proceed. I went through mm_dispersion.f90 and
understood how the forces, energy and stress were calculated. I have
been looking at the program flow (calculation and printing of
dynamical matrix) but did not get clear idea how to proceed.

Couple of queries to start?

1. Should I calculate the london contributions to dynamical matrix
separately and add it to actual matrix after that?

2. Where does the response to atomic displacement (as written in the
discussion of phonon code) calculated? If so, can we add vdwl response
also to the psuedopotential response (with a flag if london=true), and
let other programs work as they are to calculate and diagonalize the
dynamical matrix?

Any help will be appreciated.

Best Regards,

On Mon, Jun 27, 2011 at 4:15 PM, Stefano de Gironcoli <degironc at sissa.it> wrote:
> dear Vikas Varshney,
> I think you analyzed correctly the situation.
> london (grimme) correction for vdW is included in the total energy,
> force and stress but not for dynamical matrix.
> It should not be too difficult because it is an extra term in the
> energy depending explicitly on the atomic positions.
> you could have a look on how this term is computed in  energy_london,
> force_london and stress_london (all contained in
> Modules/mm_dispersion.f90)
> and work out a similar routine for dynmat_london...
> that would be a great contribution to the community.
> stefano
> -
> Stefano de Gironcoli - SISSA and DEMOCRITOS
> Quoting Vikas Varshney <vv0210 at gmail.com>:
>> 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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