Low-density expansion and isospin dependence of nuclear energy functional: Comparison between relativistic and Skyrme models

In the present work, we take the nonrelativistic limit of relativistic models and compare the obtained functionals with the usual Skyrme parametrization. Relativistic models with both constant couplings and with density-dependent couplings are considered. While some models already present very good results at the lowest order in the density, models with nonlinear terms only reproduce the energy functional if higher order terms are taken into account in the expansion.