I will explain how to make a supercell (that is, arbitrarily repeat a unit cell in space) for running simulations in VASP with a program called VESTA. VESTA can visualize the POSCAR and CONTCAR files that VASP uses to model atoms and molecules.
Download VESTA: http://jp-minerals.org/vesta/en/
In my example, I will expand a graphene unit cell to a 4×4 graphene surface. I found the POSCAR for this unit cell elsewhere online.
Open this in VESTA to visualize the graphene unit cell. I arbitrarily colored the carbon atom centered at 0,0,0 green to distinguish it from the second atom, which I left brown and is completely contained in the unit cell.
Once you have your unit cell open in VESTA and everything looks okay, proceed with the following to generate a supercell.
Edit -> Edit Data -> Unit Cell…
This will open a new window that looks like this.
This window allows you to alter your unit cell or build new unit cells from scratch. Click the “Transform…” button. This opens another window.
The rotation matrix P shown on the left allows you to multiply your unit cell along the basis vector directions, with each diagonal corresponding to the a, b, c directions. In our example, we want a 4×4 graphene unit cell, so I would simply write 4 in the first two diagonal boxes with 1’s in them. Manipulating the values in the other boxes will rotate your unit cell. Click “OK” and click yes when it asks you if you’re okay with the cell volume changing or adding new lattice points.
Voila, a 4×4 graphene unit cell!
Absolute unit. Simply in awe.
Reproducible Crucible 