  The possibility of thermal excitation of intrinsic localized modes (ILMs)
arising from anharmonicity in ionic perfect crystals is studied numerically for
realistic model systems in one and three dimensions. Implications are discussed
for an interesting high-temperature feature seen in earlier inelastic neutron
scattering experiments on single crystal NaI. The general conclusion is that
ILM formation energies are far too large for thermal excitation of ILMs to
account for the observed feature in a pure crystal.
