  Asymmetric nuclear matter is investigated in the low density region below the
nuclear saturation density. Microscopic calculations based on the Dirac
Brueckner Hartree-Fock (DBHF) approach with realistic nucleon-nucleon
potentials are used to adjust a low density functional. This functional is
constructed on a density expansion of the relativistic mean field theory which
allows a clear interpretation of the role of the mesons to the equation of
state. It is shown that a correction term should be added to the functional in
order to take into account the effects beyond the mean field. Two functionals
with different corrections are obtained. Those functionals converge to predict
a reduction of the spinodal zone in asymmetric nuclear matter by about 15-20%
and an isoscalar unstable mode closer to the constant Z/A direction than the
functional without correction.
