Symmetry of k-points and velocities

[Yan]

Dear EPW users and developers,

When I calculated the electron-phonon coupling of Si (space group 227), I used "vme = wannier" in EPW to calculate the energy, electron linewidth, and velocity of the k-point in the full BZ. I encountered two problems related to symmetry:

One is that between symmetry-equivalent k-points, the energy, line width and velocity do not meet the symmetry constraints well. This problem seems to be the cause of Wannier interpolation (see another topic for details: Electron linewidth of symmetrically degenerated kpoints)

The other problem is that velocity of the k-point does not satisfy the constraints of the k-point little co-group.

For example:
In primitive basis, the coordinates of the k-point are (0.1, 0.1, 0), and the little co-group is 4m.m, which will constrain the velocity of the point to (Vx,0,0). But the calculated result is (0.70260269, 0.01692294, -0.15118160). It can be seen that there is a significant deviation in the y and z directions, and other k-points may deviate more.

I tried a dense coarse grid (nk1=nk2=nk3= 10, nq1=nq2=nq3= 5), and I also tried "vme = dipole", and the problem still exists.

How can I get a result that satisfies the symmetry constraints?

Best,
Yan

[Zhe_L]

Dear Yan,

A well symmetry of energies or velocities relies on a well Wannierization.

We have a well-tested example of silicon in PHYSICAL REVIEW RESEARCH 3, 043022 (2021) and you can download input files from https://archive.materialscloud.org/record/2021.94

I did the same test as you described with these files and it works well. So I suggest you
1. do the same test with the example offered in the paper
2. compare it with your case

Best,
Zhe