  The energy route to the equation of state of hard-sphere fluids is
ill-defined since the internal energy is just that of an ideal gas and thus it
is independent of density. It is shown that this ambiguity can be avoided by
considering a square-shoulder interaction and taking the limit of vanishing
shoulder width. The resulting hard-sphere equation of state coincides exactly
with the one obtained through the virial route. Therefore, the energy and
virial routes to the equation of state of hard-sphere fluids can be considered
as equivalent.
