The spin-orbit force gives thus an additional contribution to the energy $$ \Delta\varepsilon_{nlj}=\frac{C}{2}\left(j(j+1)-l(l+1)-\frac{3}{4}\right), $$ which lifts the degeneracy we have seen before in the harmonic oscillator or Woods-Saxon potentials. The value \( C \) is the radial integral involving \( \xi(\boldsymbol{r}) \). Depending on the value of \( j=l\pm 1/2 \), we obtain $$ \Delta\varepsilon_{nlj=l-1/2}=\frac{C}{2}l, $$ or $$ \Delta\varepsilon_{nlj=l+1/2}=-\frac{C}{2}(l+1), $$ clearly lifting the degeneracy. Note well that till now we have simply postulated the spin-orbit force in ad hoc way. Later, we will see how this term arises from the two-nucleon force in a natural way.