An elementary classification of crystals according to their binding forces shows that an ideal crystal, defined as a perfect lattice of spherical atoms held together with two-body, central forces, should be found among the inert gas solids. The calculation of a lattice energy from an interatomic potential of the Mie-Lennard-Jones form is discussed, and the results for the hexagonal and cubic close-packed lattices compared. For argon, it is shown that a (12,6) interatomic potential, whose constants have been calculated from the crystal parameters at 0°K, can be used to predict both the specific heat of the crystal and the second virial co-efficient of the gas over suitable ranges of temperature. A comparison of this interatomic potential with existing calculations of the interaction of two argon atoms shows that the macroscopic properties of argon can be calculated at present from a pair potential, but not directly from the properties of an argon atom.