  We study glassy dynamics using a simulation of three soft Brownian particles
confined to a two-dimensional circular region. If the circular region is large,
the disks freely rearrange, but rearrangements are rarer for smaller system
sizes. We directly measure a one-dimensional free energy landscape
characterizing the dynamics. This landscape has two local minima corresponding
to the two distinct disk configurations, separated by a free energy barrier
which governs the rearrangement rate. We study several different interaction
potentials and demonstrate that the free energy barrier is composed of a
potential energy barrier and an entropic barrier. The heights of both of these
barriers depend on temperature and system size, demonstrating how non-Arrhenius
behavior can arise close to the glass transition.
