  For a large class of fluids exhibiting ultrasoft bounded pair potentials,
particles form crystals consisting of clusters located in the lattice sites,
with a density-independent lattice constant. Here we present an investigation
on the dynamic features of a representative example of this class. It is found
that particles can diffuse between lattice sites, maintaining the lattice
structure, through an activated hopping mechanism. This feature yields finite
values for the diffusivity and full relaxation of density correlation
functions. Simulations suggest the existence of a localization transition which
is avoided by hopping, and a dynamic decoupling between self- and collective
correlations.
