  We report the results of a density-functional study of the diffusion of Pt
dimers on the (111) surface of Pt. The calculated activation energy of 0.37 eV
is in {\em exact} agreement with the recent experiment of Kyuno {\em et al.}
\protect{[}Surf. Sci. {\bf 397}, 191 (1998)\protect{]}. Our calculations
establish that the dimers are mobile at temperatures of interest for adatom
diffusion, and thus contribute to mass transport. They also indicate that the
diffusion path for dimers consists of a sequence of one-atom and (concerted)
two-atom jumps.
