Dispersion energies and excitation parameters in fermion fluids

Certain physical mechanisms contributing to the Brueckner pair dispersion energy $W_2^D$ are reviewed. This $W_2^D$ 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 $W_3^D$ has not yet appeared. This $W_3^D$ depends on a triplet excitation parameter ${\kappa }_3$, which is calculated using Jastrow correlation functions. We find that ${\kappa }_3$ is small when ${\kappa }_2$ is small (≲ 0.1), and large when ${\kappa }_2$ is large (≳ 0.4). Since ${\kappa }_2\approx 0.2$ in nuclear matter, there is hope that both Brueckner and Iwamoto-Yamada expansions converge to the same answer at fairly low orders.