where r is the distance from any atom to any other atom in the simulation.
The constants 
(epsilon) and
(sigma) are important to determine the
strength and shape of the interaction and hence the properties of the
solid or liquid.
The force between individual atoms is of course, just the negative of the gradient of the potential.
To see a graph of the potential, go to the potential panel and click the "Graph" button.
Let's consider what the constants in this equation mean. Sigma describes basically where the potential equals zero. (If you plug sigma in for r you get zero.) So by changing this, you move the whole potential to the right or to the left. This can affect the overall structure of the solid state.
Epsilon describes the strength of the interaction or the depth of the
potential "well." In fact it can be shown that the depth of the well
is just 
. Try changing this
number in the potential panel and see
what effect it has on the depth of the well. This can effect many
things for the system including the freezing temperature.