[Wannier] [SUSPECT ATTACHMENT REMOVED] PW vs Wannier interpolated bandstructure of Silicon

[Jesse Vaitkus]

Hello Samuel,

unfortunately these problems are not isolated to only you. Several users
including myself have had the same problem.

I suspect most of it comes from an over eagerness to fit the points as best
as possible while getting the most localised wf.

While this sounds like what we want, only fitting the eigenvalues means
that the description between energy eigenvalues, the bands, like local
symmetry and derivatives are not conserved, imposing these properties would
likely give a less localised but more 'true' approximant.

Unfortunately I don't have a programmatic solution for the authors of w90
but I can tell you a 'hack' to get some of your fits looking better. The
first step is to realise that when your dft package plots the bands it is
actually doing a non self consistent probing of the energy landscape using
your current configuration. If you complete a run with a coarse mesh, you
can restart with a fine one (this is often great for large problems with
lattice distortions). With this in mind, calculate your converged system
with a mesh you like, then take your converged charge density, ionic
positions with a large number of kpoints.

The eigenvalues you get from this should look a lot like a regular band
plot. As w90 tries to fit all of these kpoints exactly you can force it
into a more realistic space. Not ideal but it works.

Cheers,
Jesse Vaitkus

On 12/12/2016 7:39 am, "Samuel Ponce" <samuel.ponce at materials.ox.ac.uk>
wrote:

> Dear Wannier team,
>
> I have a small question about Wannier90.
>
> Somehow I find it very difficult to get very good Wannierization of
> Silicon. I'm interested in the top of the VB and the bottom of the CB
> (between G-X).
>
> I can get okish one but when I try to improve them, I do not manage.
>
> Attach to this email is a comparison between the L-G-X path of Silicon
> with PW/bands.x (in green) VS Wannier90 in red.
>
> As you can see, the Wannier code fail to reproduce the second branch of
> the CBM (on the right), which is problematic. The wannier seems to have
> some unphysical oscillation in the CB.
>
> Attach to this email are all the file needed to obtain both plots and even
> the line needed to do the gnuplot (you have to scale them). In each of them
> I've place a README file with all the steps for convenience. I've also
> placed the psp.
> The calculations are very fast.
>
> Maybe its something very stupid.
>
> So far, I've try:
> - increasing the nscf grid (you can see a comparison between 8x8x8 and
> 10x10x10 grid. The latest 10x10x10 gives dramatic oscillations)
> - changing the number of bands
> - playing with the frozen windows
> - increasing the number of Wannier iteration
>
> Nothing made it better than that.
>
> Would you have any suggestions?
>
> PS: The lattice parameter is slightly lower (10.17 Bohr) than the DFT one
> (10.208 Bohr) on purpose but is the same in both case. In the case of the
> DFT lattice 10.208 similar issues exists but its slightly better.
>
> PPS: I also noticed that the Wannierzation was breaking the symmetries.
> For example doubly degenerate bands along G-X were slightly lifted. As a
> results, effective masses are quite bad. Is there a way to impose crystal
> symmetries?
>
> Thank you,
>
> Samuel
>
>
> _______________________________________________
> Wannier mailing list
> Wannier at quantum-espresso.org
> http://mailman.qe-forge.org/cgi-bin/mailman/listinfo/wannier
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/wannier/attachments/20161212/7c0c98fe/attachment.html>