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 ${\ensuremath{\kappa}}_{3}$, which is calculated using Jastrow correlation functions. We find that ${\ensuremath{\kappa}}_{3}$ is small when ${\ensuremath{\kappa}}_{2}$ is small (\ensuremath{\lesssim} 0.1), and large when ${\ensuremath{\kappa}}_{2}$ is large (\ensuremath{\gtrsim} 0.4). Since ${\ensuremath{\kappa}}_{2}\ensuremath{\approx}0.2$ in nuclear matter, there is hope that both Brueckner and Iwamoto-Yamada expansions converge to the same answer at fairly low orders.NUCLEAR STRUCTURE Fermion fluids, nuclear matter; compared Brueckner and IY expansions; calculated triplet excitation parameter.