Cohesive energy of covalent crystal

[helpcometk]

Homework Statement

The potential Energy per atom in a covalent crystal with interatomic spacing r may be written φ(r) =2ε [A (σ/r)^12 -B (σ/r)^ 6 ]
Derive expressions for the equilibrium interatomic spacing and the cohesive energy of the crystal.

Homework Equations

Again the problem is that i have a covalent crystal .I think cohesive energy is different for different materials, so i would like to give me only a reference (website if possible ) to look if someone knows.

The Attempt at a Solution

 

[Mapes]

Forget about the material; it's irrelevant. Given any pair potential φ(r), how do you find the equilibrium spacing? Once you have that, what's the volumetric energy difference between having a bond and not having a bond?