  Neutron drops confined in an external field are studied in the framework of
relativistic Brueckner-Hartree-Fock theory using the bare nucleon-nucleon
interaction. The ground state energies and radii of neutron drops with even
numbers from $N = 4$ to $N=50$ are calculated and compared with results
obtained from other nonrelativistic \textit{ab initio} calculations and from
relativistic density functional theory. Special attention has been paid to the
magic numbers and to the sub-shell closures. The single-particle energies are
investigated and the monopole effect of the tensor force on the evolutions of
the spin-orbit and the pseudospin-orbit splittings is discussed. The results
provide interesting insight of neutron rich systems and can form an important
guide for future density functionals.
