# Homework Help: About Lennard Jones potential.

1. Oct 18, 2015

### Helpless guy

1. The problem statement, all variables and given/known data
I have a fcc structure, the net parametre is 4.46*10-10m, the potential energy/mol is -1.88kJ/mol. My task is to find the parametres ε and σ of the potential.

2. Relevant equations

Φ=-4ε[(σ/r)6+(σ/r)12]

3. The attempt at a solution
I can find the value of r using the net paramentre; r=[(√2)/4]*a.
If I divide the potential energy/mol by 6.02*1023 I can find the potential energy/atom, but I still have only one equation with both of the parametres to determine.
I have to find another equation with one or both paramentres, but I don't know where to find it.

All sugestions are appreciated.
Thank you very much.

2. Oct 18, 2015

### Staff: Mentor

You know that this distance r in your material is at a special point of the potential. Otherwise the distance would increase or decrease.

3. Oct 18, 2015

### ehild

There is a mistake in the formula. It should be Φ=-4ε[(σ/r)6-(σ/r)12]
The neighbouring atoms are arranged at such distance from each other where the L-J potential is minimum. Find the derivative of Φ and make it equal to zero: this gives you an additional equation.

4. Oct 18, 2015

### Helpless guy

Thank you so very much! It's a brilliant idea.