  We develop a manifestly microscopic method to deal with strongly interacting
nuclear systems that have different interactions in spin-singlet and
spin-triplet states. In a first step we analyze variational wave functions that
have been suggested to describe such systems, and demonstrate that the
so-called commutator contributions can have important effects whenever the
interactions in the spin-singlet and the spin-triplet states are very
different. We then identify these contributions as terms that correspond, in
the language of perturbation theory, to non-parquet diagrams. We include these
diagrams in a way that is suggested by the Jastrow-Feenberg approach and show
that the corrections from non-parquet contributions are, at short distances,
larger than all other many-body effects.
