How to make up radius line on molecular dynamic simulation?

I'm working on the following problem (molecular dynamic simulation) and would like some help: Chromium is an element that is widely used as a component of biomedical implant materials. The mass of the Cr atom is 51.99, the atomic radius is 128pm, the crystal structure is BCC, the melting point is 1907 C. The molecular dynamics simulation or atomic dynamics for Cr with the number of atoms 2000, the potential is approached by Lennard Jones model with ε = 0,23 eV and δ = 2,34 A with step= 0,04 and nstep = 5000

Question:

a. What is the total energy if lattice constant is 0,92a; 0,96a; a; 1,04a ; 1,08a?

b. Plot the total energy vs volume supercell (V), and determine fitting function.

I've been done this far, but I dont know how to determine in radius part before '90 90 90 3 3 3'.

My work:

For sys.specs file, I've been done this far:

Chromium 2000
1 0 0 0 51.99 0 Cr
end
Lennard-Jones
1 1 0.23 2.34
end
%I dont know what I should fill in here 90 90 90 3 3 3
Chromium -0.5 0.5 0.5
Chromium 0.5 -0.5 0.5
Chromium 0.5 0.5 -0.5
end 

My problems are:

1. I dont know how to decide in 'radius' part.
2. Where should I put lattice constant 0,92a; 0,96a; a; 1,04a ; 1,08a?
3. Where can I find volume super cell and its energy?

Thank you.