[Wannier] vasp-wannier supercell symmetry

[Aron Szabo]

Dear Wannier users,

I was trying to do some calculations with supercells, and I've noticed 
that wannier90 won't converge when the supercell has different symmetry 
than the primitive unit cell. E.g. on a graphene sheet using 4 atoms in 
a cell instead of 2, I can either define the new lattice vectors just as 
multiplying one of the original ones with 2, or I can set orthogonal 
lattice vectors as well. In vasp it won't make any difference in the 
energies, and also the band structures can be mapped onto each other 
just considering the different shapes of the Brillouin zone. But while 
in the first case wannier90 will produce the same Wannier functions as 
in a primitive cell calculation (just repeating them on the other two 
atoms), in the second case it won't converge to localized results. 
Instead of the order of 1, the spreads are in the order of 10 or 100, 
when starting from the same projections, and doing a few hundred iterations.

The same happened when building up supercells of zinc-blende GaSb. When 
keeping the symmetry of the unit cell it works fine, but when building 
an orthorombic cell, it doesn't converge. First I got negative weights 
for the k point neighbours, and negative spreads at the end. I found out 
that it was because instead of just taking the first two neighbours 
along each axis (as it should be for orthorombic cells) four other k 
points (along diagonal directions) were taken too, because they were 
exactly at the same distance as the first neighbors along the axial 
direction (the sides of the BZ are a, b, and c, with c^2 = a^2+b^2, due 
to the symmetry of the system). Artificially changing one lattice vector 
a bit solved this part with the negative weights, but still, the Wannier 
functions did not converge to a localized state, but the spread remained 
huge.

Has anyone met such issues, and had maybe any idea what causes this, and 
how to solve it, when the use of a supercell with some different 
symmetry as the primitive cell can't be avoided (e.g. to simulate an 
ultra-thin-body transistor with some pre-defined transport orientation)?

Thank you in advance for any help

Aron Szabo
PhD student @ ETH Zurich, Integrated Systems Laboratory