  The spin symmetry in the Dirac sea has been investigated with relativistic
Brueckner-Hartree-Fock theory using the bare nucleon-nucleon interaction.
Taking the nucleus $^{16}$O as an example and comparing the theoretical results
with the data, the definition of the single-particle potential in the Dirac sea
is studied in detail. It is found that if the single-particle states in the
Dirac sea are treated as occupied states, the ground state properties are in
better agreement with experimental data. Moreover, in this case, the spin
symmetry in the Dirac sea is better conserved and it is more consistent with
the findings using phenomenological relativistic density functionals.
