The curve labeled [*no structure*]{} means the case where $Z\rho_{n}(r)=N\rho_{p}(r)$ and the other two curves are obtained within the TF approximation and the full solution of the Dirac equation in the Hartree approximation. At the momentum transfer values of recent experimental interest (around $\simeq 0.4$ fm$^{-1}$), the curves are almost identical. A careful analysis of the same results in a different scale shows us that the asymmetry changes $12$ and $11$ percent respectively within the Dirac and TF approximations in comparison with the [*no structure*]{} case. Since it is the measurement of the asymmetry in this low momentum transfer region that will provide the accurate result for the neutron skin thickness, we have restricted our calculations to the TF approximation, as stated in the Introduction.

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   ![Parametrization HS, comparison Thomas-Fermi-HO versus Dirac-HO[]{data-label="asym0"}](fig4a.eps "fig:"){width="8.cm"}  
   ![Parametrization HS, comparison Thomas-Fermi-HO versus Dirac-HO[]{data-label="asym0"}](fig4b.eps "fig:"){width="8.cm"}  
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In Fig. \[asym2\]a we show the asymmetry obtained with the NL3 model for both numerical calculations in the TF approximation, i.e, nucleation and HO expansion methods. In this case, the agreement is very satisfactory even for larger $q$-values, although the small numerical discrepancies is reflected in a $\sim 10$ percent difference in the predicted neutron skin thickness, 