Hi Anton
I have another error with zigzag ribbons. When I run graphene lattice with a complex next-nearest-neighbors (Kane-Mele model). I have the next error for values "t2<0.0005j":

RuntimeError: Numbers of left- and right-propagating modes differ, possibly due to a numerical instability.

If somebody know how fix it, please let me know. 
Best Regards

2015-08-05 11:50 GMT-03:00 Anton Akhmerov <anton.akhmerov@gmail.com>:
Hi Maximilian, César,

Thanks for reporting this, looks quite interesting. I don't have an
immediate answer yet, but I'll check it out. Can you try and search
for the simplest/smallest system that shows the instability?


On Wed, Aug 5, 2015 at 4:45 PM, cesar nunez
<cesar.nunez@postgrado.usm.cl> wrote:
> Dear All
> I have the same problem when used the Kane-Mele model. The problem
> disappears when the next-nearest-neighbor hopping is zero or lower than
> 0.0005j. Some ideas for fix it?
> Best Regards
> César
> 2015-08-05 6:08 GMT-03:00 Maximilian Trescher
> <maximilian.trescher@fu-berlin.de>:
>> 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
> --
> César Núñez Ramírez
> Doctor(C) Física
> U.T.F.S.M.

César Núñez Ramírez
Doctor(C) Física