  We present a numerical investigation of the Brownian motion and diffusion of
a dumbbell in a two-dimensional periodic potential. Its dynamics is described
by a Langevin model including the hydrodynamic interaction. With increasing
values of the amplitude of the potential we find along the modulated spatial
directions a reduction of the diffusion constant and of the impact of the
hydrodynamic interaction. For modulation amplitudes of the potential in the
range of the thermal energy the dumbbell diffusion exhibits a pronounced local
maximum at a wavelength of about 3/2 of the dumbbell extension. This is
especially emphasized for stiff springs connecting the two beads.
