boundary x y z
p is periodic n is non-periodic
boundary p p n
Set the periodicity of the global domain in each of the x,y,z dimensions. "N" means the domain is non-periodic in that dimension, so that if particles move outside the domain, they are lost. "P" means the domain is periodic in that dimension, so particles wrap-around to the other size and can react with nearby particles across the periodic boundary.
All dimensions are periodic.