Go to File ⇒ Import ⇒ Crystal and select aluminum.
Go to Crystal => Space Group => Fill Unit Cell (this will shortly be un-necessary)
Okay… how many atoms are there in the unit cell?
- There are 8 corner atoms, each 1/8 in the cell
- There are 6 face-centered atoms, each 1/2 in the cell
So that’s 4 atoms per unit cell. You wanted 64 atoms. So that’s 4^3 atoms. If we double along the a-axis, we get 8 atoms. If we double that along the b-axis (i.e., 2x2x1 super-cell) we get 16 atoms. And if we double along the c-axis, we get 32 atoms.
So it’s a little tricky to get 64 Al atoms in a cube. Let’s go with a 2x2x3 super cell, which at least gives us the 64 atoms you want.
I’m not sure how you’d want to handle that - do you absolutely need a cube?