# Re: [Kwant] Symmetry and leads

```Dear David,

Currently a Kwant symmetry stores the extra directions that describe
how it handle various lattices. If you don't do anything, the choice
of the extra directions happens automatically, and a Bravais lattice
vector is chosen. However you can manually override this by using the
add_site_family method of the TranslationalSymmetry (see
for details). I know several people have used this to solve exactly
the problem that you currently encounter.```
```
Best,
Anton

On Thu, Oct 23, 2014 at 4:40 PM, David Abergel
<david.aber...@nordita.org> wrote:
> Dear all,
>
> I am having a problem defining a system which matches what I want.
>
> I start by defining a rectangular graphene lattice of size 0<=x<L in the x
> direction and 0<=x<W in the y direction. (By "rectangular" I mean in the
> real space coordinates, not the crystallographic coordinates.)
>
> I want to attach a lead to the left-had end of this rectangle, going to
> minus infinity. Therefore, I define a lead with translational symmetry
> (-1,0) and the appropriate hopping. I attach the lead and plot the system.
>
> When I plot the system, I find that the lead has been attached along the
> (0,1) crystallographic direction (so, that is along the (1/2, sqrt(3)/3)
> real space vector). A triangle of extra sites have been added for x<0 (real
> space) so that the total shape of the scattering region is now not
> rectangular.
>
> If I attach another lead with lead.reversed(), a similar thing happens on
> the right of the sample so that my scattering region is now a parallelogram.
>
> As I understand it, this should not affect the physics in any way, since the
> lead is semi-infinite. But, if I want to draw pictures, plot functions over
> the scattering region, and gain physical understanding, it is a bit of a
> pain. So, my question is whether there is any way to make the lead attach
> along the (0,1) real space direction (which is the same as the (-1,2)
> crystallographic direction) and yet maintain the (-1,0) translational
> symmetry?
>
> If you require a sample program which reproduces this behavior then I can
> easily provide that, but I thought I should not extend an already long post
> unnecessarily.
>