  We investigate the accuracy of the expression of Rosenfeld and Tarazona (RT)
for the excess isochoric heat capacity, C_V^{ex} \propto T^{-2/5}, for eighteen
model liquids. Previous investigations have reported no unifying features of
breakdown for the RT expression. Here liquids with different stoichiometric
composition, molecular topology, chemical interactions, degree of undercooling,
and environment are investigated. We find that the RT expression is a better
approximation for liquids with strong correlations between equilibrium
fluctuations of virial and potential energy, i.e., Roskilde simple liquids
[Ingebrigtsen et al., Phys. Rev. X 2, 011011 (2012)]. This observation holds
even for molecular liquids under severe nanoscale confinement, the physics of
which is completely different from the original RT bulk hard-sphere fluid
arguments. The density dependence of the specific heat is predicted from the
isomorph theory for Roskilde simple liquids, which in combination with the RT
expression provides a complete description of the specific heat's density and
temperature dependence.
