Periodic Boundary Conditions

[hsuya]

I am working on a 2D material. How do I incorporate the periodic boundary conditions when trying to relax the structure and perform other caclulations? I saw in one of the tabs for Poisson Solver that there was written periodic boundary conditions as shown in picture 1 and picture 2.
When changing the solver type to FFT I could not modify anything in the dropdowns for Boundary Condition and when selecting conjugate gradient I am only getting Dirichlet and Periodic options.
So I want to ask
1. How to select periodic boundary conditions?
2. To prevent periodic interaction between repeating layers in c-direction we add a vacuum spacing so, instead of that could we use a different boundary condition which might help us to decrease the vacuum spacing and save us some time computationally?

[mlee]

What is your 2D model? Is it bulk or slab or device? GUI  can be automatically shown based on your model.
 
You can see the default boundary condition for each type of model.
https://docs.quantumatk.com/manual/Types/FastFourier2DSolver/FastFourier2DSolver.html

[hsuya]

I am working on monolayer of TMDs. Yes but I am not able to understand that if I don't want periodicity in c/z direction should I use Dirichlet condition? I want to keep periodicity in just x(a) and y(b) directions.

[Petr Khomyakov]

To avoid using periodic boundary conditions, you may adopt Dirichlet-Dirichlet (or Neumann-Neumann) boundary conditions in the out-of-plane z-direction, or mixed Dirichlet-Neumann boundary conditions if the 2D system of study is asymmetric (no mirror symmetry) in z-direction. In this case, one may avoid an artificial dipole formation due to this asymmetry.