Answer
The xy-plane of the simulation box should exactly fit the xy-plane of the slab whereas the z-dimension should be considerably larger than the slab thickness. The slab can then be placed midpoint along the z-direction.
The xy-plane of the simulation box should exactly fit the xy-plane of the slab whereas the z-dimension should be considerably larger than the slab thickness. The slab can then be placed midpoint along the z-direction.