Material conditions on an external boundary


I am trying to run a simulation where one surface of the computational domain boundary has material properties. If I have a cubic domain and want 5 sides to be the external boundary (for the plasma) and one side, the bottom for example, to have spacecraft surface properties (i.e. being the elecNode-00 ground, or having material properties), how can I go about this? Does the external boundary need to have closed surfaces? I effectively want to simulate a “metal plate” as one section of the boundary, and have the rest be plasma bounds.

I get an error when I try to initialize the simulation:
spis.Util.Exception.SpisRuntimeException: java.lang.ArrayIndexOutOfBoundsException: 0 at spis.Solver.ElectroMag.Poisson.PotPoissonSolver.quicksort(

There is no need to have a close surface for the external boundary. The metal plate case should work.
The quiksort error at this line should only occur if you did not define a computational volume.