  We apply a particle-particle RPA model to study the properties of the
two-neutron valence wave function in nuclei 14-C, 12-Be, 11-Li and 14-Be. The
RPA model takes account of two body correlations in the core so that it gives a
better description of energies and amplitudes than models which assume a
neutron closed shell (or subshell) core. With a Gogny neutron-neutron effective
interaction or with the equivalent density dependent delta force we are able to
reproduce the two-neutron separation energies in these nuclei and in the
corresponding cores, except for 9-Li. These calculations suggest the same
2s-1p1/2 shells inversion in 12-Be_13-Be than in 11-Be.
