  The Uhlenbeck-Ford (UF) model was originally proposed for the theoretical
study of imperfect gases, given that all its virial coefficients can be
evaluated exactly, in principle. Here, in addition to computing the previously
unknown coefficients B11 through B13, we assess its applicability as a
reference system in fluid-phase free-energy calculations using molecular
simulation techniques. Our results demonstrate that, although the UF model
itself is too soft, appropriately scaled Uhlenbeck- Ford (sUF) models provide
robust reference systems that allow accurate fluid-phase free-energy
calculations without the need for an intermediate reference model. Indeed, in
addition to the accuracy with which their free energies are known and their
convenient scaling properties, the fluid is the only thermodynamically stable
phase for a wide range of sUF models. This set of favorable properties may
potentially put the sUF fluid-phase reference systems on par with the standard
role that harmonic and Einstein solids play as reference systems for
solid-phase free-energy calculations.
