  The long-standing controversy about the isospin dependence of the effective
Dirac mass in ab initio calculations of asymmetric nuclear matter is clarified
by solving the relativistic Brueckner-Hartree-Fock equations in the full Dirac
space. The symmetry energy and its slope parameter at the saturation density
are $E_{\text{sym}}(\rho_0)=33.1$ MeV and $L=65.2$ MeV, in agreement with
empirical and experimental values. Further applications predict the neutron
star radius $R_{1.4M_\odot}\approx 12$ km and the maximum mass of a neutron
star $M_{\text{max}}\leq 2.4M_\odot$.
