Hi Pierre-Antoine,
You are close, but try:
units metal
dimension 3
boundary p p p
atom_style atomic
variable xx equal 3
Atom definition
lattice diamond 5.44
region box block 0 {xx} 0 {xx} 0 ${xx}
create_box 2 box
create_atoms 1 box basis 1 1 basis 2 1 basis 3 1 basis 4 1 basis 5 2 basis 6 2 basis 7 2 basis 8 2
Force fields
mass 1 28.0855
mass 2 12.01…
Please change the lattice parameter!
Javier