  We present a direct calculation by molecular-dynamics computer simulation of
the crystal/melt interfacial free energy, $\gamma$, for a system of hard
spheres of diameter $\sigma$. The calculation is performed by thermodynamic
integration along a reversible path defined by cleaving, using specially
constructed movable hard-sphere walls, separate bulk crystal and fluid systems,
which are then merged to form an interface. We find the interfacial free energy
to be slightly anisotropic with $\gamma$ = 0.62$\pm 0.01$, 0.64$\pm 0.01$ and
0.58$\pm 0.01 k_BT/\sigma^2$ for the (100), (110) and (111) fcc crystal/fluid
interfaces, respectively. These values are consistent with earlier density
functional calculations and recent experiments measuring the crystal nucleation
rates from colloidal fluids of polystyrene spheres that have been interpreted
[Marr and Gast, Langmuir {\bf 10}, 1348 (1994)] to give an estimate of $\gamma$
for the hard-sphere system of $0.55 \pm 0.02 k_BT/\sigma^2$, slightly lower
than the directly determined value reported here.
