  The Kadanoff-Baym (KB) equations are solved numerically for infinite nuclear
matter. In particular we calculate correlation energies and correlation times.
Approximating the Green's functions in the KB collision kernel by the free
Green's functions the Levinson equation is obtained. This approximation is
valid for weak interactions and/or low densities. It relates to the extended
quasi-classical approximation for the spectral function. Comparing the
Levinson, Born and KB calculations allows for an estimate of higher order
spectral corrections to the correlations. A decrease in binding energy is
reported due to spectral correlations and off-shell parts in the reduced
density matrix.
