I want to run molecular dynamics with LAMMPS, the aim is to simulate a platinum surface with a gas phase consisting of simple molecules. I would like to use periodic boundary conditions in order to create a bulk-like behaviour for the gas phase. However, I'm not sure how to place the surface, I see two obvious options:
I place the surface at the middle of the simulation box (It will split the simulation box in two then) and ask for PBCs in all the directions of the space.
I place the surface at the bottom of the simulation box and remove the periodicity along the axis perpendicular to the surface and constrain it to stay there.
The first option seems to be the best but requires the surface to be thick enough. The second option is less physical as one direction is not well represented and constraints are applied. I still have doubt because for ab-initio calculations (Single points) I used to choose the second one.