In practice, pair potentials are cut off at a certain range, which can have a surprising effect on stability as shown in Figure 1

While the LJ fluid is very well studied, the finite temperature crystal structure has only recently been resolved. The problem is that the fcc and hcp structures are extremely close in energy (see Figure 1(b)), so the entropy must be calculated extremely accurately.

This has been done by Jackson using ‘lattice — switch Monte Carlo3 (see Figure 2). The equivalent phase diagram for the Morse potential remains unsolved. The LJ potential has been used extensively for fcc materials, and it still comes as a surprise to many researchers that fcc is not the ground state.