  Molecular dynamics simulations are used to investigate the atomic mobility
and diffusivity of a generalized Frenkel-Kontorova model which takes into
account anharmonic (exponential) interaction of atoms subjected to a
three-dimensional substrate potential periodic in two dimensions and nonconvex
(Morse) in the third dimension. The numerical results are explained by a
phenomenological theory which treats a system of strongly interacting atoms as
a system of weakly interacting quasiparticles (kinks). Model parameters are
chosen close to those for {\bf K}-{\bf W}(112) adsorption system.
