[QE-users] any tool to get real space force constant from shifted q grid or manually q grid phonon calculations?

[Lorenzo Paulatto]

> Yes, I understand the cost of phonon is the critical part.
> For clarification, I meant if I perform phonon calculation on less q
> points (not on full gamma centered MP grids),
>

This is not how force constants work: as they are the (discrete) Fourier
transform of the dynamical matrix, they can only be computed accurately for
points which are reciprocal to the q-points in the grid.

Using shifted grids is a good trick when computing "square modulus"
integrals, where the phase does not count. But the calculation of force
constants is all about the phase, and shifted grids do not work very well
for this case.

The fact that you eventually want the DOS is a red herring, it is
completely irrelevant to the problem.

Kind regards

how could I get reliable real space force constants toward the calculation
> of DOS.
>
> Yi Wang
> Nanjing University of Sci. & Tech.
> On 5/10/2018 12:42，Lorenzo Paulatto<paulatz at gmail.com> <paulatz at gmail.com>
> wrote：
>
> Hello,
> the cost of computing the phonon dos with matdyn is zero, compared to the
> cost of the phonon calculation itself.
>
> --
> Lorenzo Paulatto
> Written on a virtual keyboard with real fingers
>
> On 10 May 2018 06:31, "Yi Wang" <wangwy13 at gmail.com> wrote:
>
> Dear everyone,
>
> As I understand, the q2r.x works only on gamma centered q grids for the
> DFPT calculations. However, is there any tool to calculate the real space
> force constant for the shift q points calculations and the manually q grid
> calculations?
>
> If it is possible, I want to test the phonon DOS calculation on some
> smaller q grids for the sake of saving cost.
>
> Thank for your guidance.
>
> Yi Wang
> Nanjing University of Sci. & Tech.
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