The microscopic diffusion mechanisms of a rigid, homonuclear dimer adsorbed on a fcc(100) crystal surface have been studied by utilizing static minimum-potential-energy calculations. The Lennard-Jones potential is used to model the physical interaction between the dimer atoms and the surface atoms. By variation of either the dimer length or the diameter of the dimer atoms relative to the surface lattice constant, several interesting and complicated diffusion mechanisms have been found to exist. We discuss some implications of our findings.