QuantumATK Forum

QuantumATK => General Questions and Answers => Topic started by: qweasel on February 13, 2017, 23:54

Title: Poisson solver settings for Electronic Transport Through A Distorted Nanoribbon
Post by: qweasel on February 13, 2017, 23:54

In the video "Electronic Transport Through A Distorted Nanoribbon (https://www.youtube.com/watch?v=t3xJNqP4X04)", the settings of the Poisson solver are kept as default (FFT, periodic boundary conditions).

Why is that? If it's a ribbon, wouldn't more suitable boundary conditions be non-periodic?

Title: Re: Poisson solver settings for Electronic Transport Through A Distorted Nanoribbon
Post by: Anders Blom on February 14, 2017, 00:46

Sure, but that's actually the default for devices (Dirichlet in the transport direction).

Title: Re: Poisson solver settings for Electronic Transport Through A Distorted Nanoribbon
Post by: qweasel on February 14, 2017, 01:10

In the transport direction it is indeed Dirichlet. I was referring to the top/bottom direction. Why keep it as periodic in the case of a nanoribbon?

Title: Re: Poisson solver settings for Electronic Transport Through A Distorted Nanoribbon
Post by: Anders Blom on February 14, 2017, 19:57

Why not? FFT is a lot faster than the alternatives, and as long as you have enough vacuum, any boundary condition works just as fine. Of course, if you actually have a gate in any direction, then it changes, but forcing the field to be zero if the vacuum is not large enough (and no gate present) is actually more wrong than a periodic solution which at least would have zero derivative at the edge of the cell (ok, so would Neumann, but you can only place that on one side or else it's equivalent to periodic :) ). So in short, FFT is the default basically because it's not wrong, and faster.