  We study surface diffusion in the framework of a generalized
Frenkel-Kontorova model with a nonconvex transverse degree of freedom. The
model describes a lattice of atoms with a given concentration interacting by
Morse-type forces, the lattice being subjected to a two-dimensional substrate
potential which is periodic in one direction and nonconvex (Morse) in the
transverse direction. The results are used to describe the complicated
exchange-mediated diffusion mechanism recently observed in MD simulations [J.E.
Black and Zeng-Ju Tian, Phys. Rev. Lett. {\bf 71}, 2445-2448(1993)].
