  We study autocorrelation functions of energy, heat and entropy densities
obtained by molecular dynamics simulations of supercritical Ar and compare them
with the predictions of the hydrodynamic theory. It is shown that the predicted
by the hydrodynamic theory single-exponential shape of the entropy density
autocorrelation functions is perfectly reproduced for small wave numbers by the
molecular dynamics simulations and permits the calculation of the
wavenumber-dependent specific heat at constant pressure. The estimated
wavenumber-dependent specific heats at constant volume and pressure, $C_{v}(k)$
and $C_{p}(k)$, are shown to be in the long-wavelength limit in good agreement
with the macroscopic experimental values of $C_{v}$ and $C_{p}$ for the studied
thermodynamic points of supercritical Ar.
