Dispersion energies and excitation parameters in fermion fluids

Certain physical mechanisms contributing to the Brueckner pair dispersion energy W/sup D//sub 2/ are reviewed. This W/sup D//sub 2/ is found to appear also in the Iwamoto-Yamada cluster expansion if the same Brueckner state- and momentum-dependent correlation operators are used. It does not appear in the two-body Iwamoto-Yamada energy, however, but in the three-body energy. Up to this point, a Brueckner triplet dispersion energy W/sup D//sub 3/ has not yet appeared. This W/sup D//sub 3/ depends on a triplet excitation parameter kappa/sub 3/, which is calculated using Jastrow correlation functions. We find that kappa/sub 3/ is small when kappa/sub 2/ is small (approximately-less-than0.1), and large when kappa/sub 2/ is large (approximately-less-than0.4). Since kappa/sub 2/ approx. = 0.2 in nuclear matter, there is hope that both Brueckner and Iwamoto-Yamada expansions converge to the same answer at fairly low orders.