The existence of anharmonic localization of lattice vibrations in a
perfect 3-D diatomic ionic crystal is established for the rigid-ion
model by molecular dynamics simulations. For a realistic set of NaI
potential parameters, intrinsic localized gap modes vibrating in the
 and  directions are observed for the fcc and zinc blende lattices.
An axial elastic distortion is an integral feature of this mode which
forms more readily for the zinc blende than for the fcc structure.
Molecular dynamics simulations verify that in each structure this
localized mode may be stable for at least 200 cycles.
The interchange of the IGM vibration between  and 
directions is expected, for example, with hindered rotational motion of the
excitation about the lattice site. This concomitant low frequency
component of the IGM may provide new experimental ways to excite and
identify these nonlinear excitations.
In order to identify the eigenvectors of these strongly anharmonic
localized modes, an artificial dynamical simulated annealing technique
of the Car-Parrinello-type has been developed. First, the nonlinear
difference equations for the particles' vibrational amplitudes up to
second harmonic in frequency are obtained self-consistently. Then, the
resulting system of up to 9,000 nonlinear equations is solved
successfully using this method. This numerical technique is
general enough to apply to any nonlinear system which allows a
classical molecular dynamics treatment, including the rigid-ion and
Last modified: April 12, 1997