Dear kwant users and developers,

this is my first post to this list, I'm a PhD student from Berlin.
Currently I'm trying to implement a lattice model of a pyrochlore slab
in kwant.

I found a (more or less straightforward) way to introduce periodic
boundary conditions in the system in the maillinglist archive.
This works very nicely as long as I keep the periodicity along a lattice
vector.

But I run into problems when I choose different directions: Most of the
time all solvers fail with a
"LinAlgError: QZ iteration failed to converge in zgges".

I know about this post
http://comments.gmane.org/gmane.comp.science.kwant.user/372
but in my case it occurs not only for a band bottom, but for a range of
energies.


To give you all the details I prepared a Ipython notebook that you can
view here:
http://nbviewer.ipython.org/gist/faucon/729050da5e3f2eae1bb8#

(Note that the inclusion of complex next-nearest-neighbor hopping is
crucial to produce the error, I tried to create the shortest example
which reproduces my problem).

Do you have any idea how to circumvent this problem?
Is it somehow related to
http://netlib.org/lapack/Errata/vrac/lapack_known_issues.html#QZ this
problem in Lapack?

I would be thankful for any hint,
Best,
Maximilian