[lammps-users] hcp 111 gold surface, how to define best to get the smallest periodic unit?

Dear all,
I’m at a bit of an impasse with a problem regarding 1 1 1 surfaces: I tried to create a 1 1 1 surface by using the fcc lattice and rotating it in order to align the 1 1 1 face with the xy plane and one of the vectors of the hexagonal surface lattice with the y direction

lattice fcc 4.08 orient x 2 -1 -1 orient y 0 1 -1 orient z 1 1 1

to create a periodic box of minimum periodicity I need to set the x size as 3 in lattice coordinates.
I have to deposit a superlattice with a periodicity of 4 by 7 in terms of hexagonal ssurface vectors unfortunately that means that I have to define a very large periodic box in the x direction to get both periodicity of the substrate and the superlattice to be correct.

Am I going at this the wrong way? Is there a better way to define the simulation box so that I get a smaller system that satisfies periodicity?

Thank you,
Mattia Siviero

If you have incommensurate lattices and systems where the box size for exact periodicity is very large, you may have to consider tweaking the lattice constants a little bit.
However, this will come with some margin of error, thus you will have to decide which error is dominant in your model.