We can use these results to attempt our first link with experimental data, namely to compute the shell gap or the separation energies. The shell gap for neutrons is given by $$ \Delta S_n= 2BE(N,Z)-BE(N-1,Z)-BE(N+1,Z). $$ For \( {}^{16}\mbox{O} \) we have an experimental value for the shell gap of \( 11.51 \) MeV for neutrons, while our Hartree-Fock calculations result in \( 25.65 \) MeV. This means that correlations beyond a simple Hartree-Fock calculation with a two-body force play an important role in nuclear physics. The splitting between the \( 0p_{3/2}^{\nu} \) and the \( 0p_{1/2}^{\nu} \) state is 4.88 MeV, while the experimental value for the gap between the ground state \( 1/2^{-} \) and the first excited \( 3/2^{-} \) states is 6.08 MeV. The two-nucleon spin-orbit force plays a central role here. In our discussion of nuclear forces we will see how the spin-orbit force comes into play here.