  Properties of nuclear systems at subsaturation densities can be obtained from
different approaches. We demonstrate the use of the density autocorrelation
function which is related to the isothermal compressibility and, after
integration, to the equation of state. This way we connect the Landau Fermi
liquid theory well elaborated in nuclear physics with the approaches to dilute
nuclear matter describing cluster formation. A quantum statistical approach is
presented, based on the cluster decomposition of the polarization function. The
fundamental quantity to be calculated is the dynamic structure factor.
Comparing with the Landau Fermi liquid theory which is reproduced in lowest
approximation, the account of bound state formation and continuum correlations
gives the correct low-density result as described by the second virial
coefficient and by the mass action law (nuclear statistical equilibrium). Going
to higher densities, the inclusion of medium effects is more involved compared
with other quantum statistical approaches, but the relation to the Landau Fermi
liquid theory gives a promising approach to describe not only thermodynamic but
also collective excitations and non-equilibrium properties of nuclear systems
in a wide region of the phase diagram.

Nuclear Theory