Variational and parquet-diagram calculations for neutron matter. III. S -wave pairing

We apply parquet-diagram summation methods for the calculation of the superfluid gap in S-wave pairing in neutron matter for realistic nucleon-nucleon interactions such as the Argonne v6 and the Reid v6 potentials. It is shown that diagrammatic contributions that are outside the parquet class play an important role. These are, in variational theories, identified as "commutator contributions."Moreover, using a particle-hole propagator appropriate for a superfluid system results in the suppression of the spin-channel contribution to the induced interaction. Applying these corrections to the pairing interaction, our results agree quite well with quantum Monte Carlo data.