  We propose a free-energy based Monte-Carlo method to measure the volume of
potential-energy basins in configuration space. Using this approach we can
estimate the number of distinct potential-energy minima, even when this number
is much too large to be sampled directly. We validate our approach by comparing
our results with the direct enumeration of distinct jammed states in small
packings of frictionless spheres. We find that the entropy of distinct packings
is extensive and that the entropy of distinct hard-sphere packings must have a
maximum as a function of packing fraction.
