Purpose

Module for Lennard-Jones pair potential.

$$\nonumber V(r) = 4 \epsilon \left[ \left( \frac{\sigma}{r} \right)^{12} - \left( \frac{\sigma}{r} \right)^6 \right]$$

For parameters see Ashcroft and Mermin, Solid State Physics.

The IPModel_LJ object contains all the parameters read from a LJ_params XML stanza.