Role of single-particle spectrum in the ring-diagram approach for nuclear matter

A previously developed ring-diagram formalism, which substantially improves the predictions for the saturation density of nuclear matter, is employed for different choices for the single-particle (s.p.) spectrum. Three types of s.p. spectra are studied: a gap spectrum, a continuous choice, and a model-space spectrum. It is found that the choice of the s.p. spectrum has some influence on the saturation energy, but little influence on the saturation density. Thus, the main virtue of the ring-diagram approach turns out to be independent of the choice of the s.p. spectrum. We also give a clear demonstration of the strongly density-dependent ring effect and compare our results with those obtained using other many-body methods