creation simulation box

Dear Lammps Users & Developpers
I have a problem to create simulation box that contain two materials with two different lattice constant.
1- because of using periodic boundary condition in X and Y direction and avoid of overlapping of atoms in boundaries, I prefer to use “units lattice” to create the regions (simulation region, al region and fe region).
1- I created the simulation box and region for “Fe” with “Fe” lattice constant and then try to create “Al” region smaller with “Al” lattice constant, but due to gap between the Al atoms and simbox boundary , I don’t have periodic boundary behavior there.
2- I tried to merge two region with diffrent size (X & Y direction) to create simulation box but according to this approach a step would created in interaction of to region and because of atom moving in Z direction during simulation, I think it is not a good idea( I am not sure).
I know , I can calculate and use “units box” instead of “units lattice” but I couldn’t do it without overlapping and have not good behavior at periodic boundaries.
Please any valuable advise.
Thanks in advance

lattice fcc 3.515 origin 0.5 0.0 0.0
region simulationbox block 0 4 0 4 0 4
create_box 2 simulationbox
region Febox block 0 4 0 4 0 2

regionAlbox block 0 4 0 4 2 4

create_atoms 1 region Febox

lattice fcc 4.05 origin 0.5 0.0 0.0

create_atoms 2 region Albox

Try this out: It’s for GaN nanowire simulation…

You can only create a combined system for commensurate respective lattice constants and box dimensions.

Example: if you have a iron lattice constant of 3 and an aluminum lattice constant of 4 then you have a commensurate system if you create your system box with either 4 units of the iron lattice or 3 units of the aluminum lattice or multiples of those.

With your numbers you would need a very large system for an exact match, so you need to make an approximation and adjust your lattice constants and box size until you have an acceptable match. You may need to make adjustments anyway since models rarely represent experimental lattice constants and for as thin layers as you have, there will be reconstruction throughout.
For the z direction you don't have to use integer dimensions, of course.