  The Dirac structure of the nucleon self-energy in symmetric nuclear matter as
well as neutron matter is derived from a realistic meson exchange model for the
nucleon-nucleon (NN) interaction. It is demonstrated that the effects of
correlations on the effective NN interaction in the nuclear medium can be
parameterized by means of an effective meson exchange. This analysis leads to a
very intuitive interpretation of correlation effects and also provides an
efficient parametrization of an effective interaction to be used in
relativistic structure calculations for finite nuclei.
