I want to build a montmorillonite slit pore like picture(a) following. Considering period boundary condiction on z direction, if there exists repulsion among lower atoms nearby bottom of system and upper atoms nearby top of system ? And whether the model like picture(b) can be regarded as slit pore like (a) when considering the period boundary condition on z direction or not ? Could you give me some suggestions about period boundary condition in lammps? Thank you very much.