what kind of Muti-grid should be choosen when calcult graphene_bulk transmission

[hzkvictory]

http://quantumwise.com/publications/tutorials/mini-tutorials/167
shows the way to calculate the transmission of grpahene_bulk.
   my question is: when a gate voltage is applied to the graphene sheet, ie, in the spatial region define metallic region and dielectric region, what kind of multi-grid in Poission Solver should be choosen?
   In fact, I have done many on the calculations  to get the right choice, but failed.
Attached are the scripts.

[hzkvictory]

when metallic and dielectric region are defined, it seems Neumann condition should be used,then I use Neumann in A,B directions, and Periodic in C direction, however, the result get worse

[Nordland]

All the results are "correct", but it is the periodic you want.

When you set the boundary condition to be non-periodic, there is no mirror atoms in the other cells, and hence you are effectively creating a 1D system,
and with a perfect 1D system you get a perfect step transmission spectrum.

If you want the to model the graphene bulk properties you need to make the structure periodic, and therefore in your case you should choose periodic in the a-axises. What you choose in the b-axis shouldn't really matter in this simulation, however I would go with Neumann.

[Nordland]

Oh by the way, if you are wondering why(if) you are not seeing any effect of the gate, it is due that when you have a gate like you have, you might end up with just shifting the energy zero, and not the bands.

[abhi005]

please suggest me what type of multigrid i have to choose for CNT fet.I have choosen neumann but it shows infnity error then i choose drichlet for B Bottom again error is quite large in the range of e+000.i am also attaching .py file.please help me out of this problem.