  We are reinvestigating the hyperfine structure of sodium using a fully
relativistic multiconfiguration approach. In the fully relativistic approach,
the computational strategy somewhat differs from the original nonrelativistic
counterpart used in J\"onsson et al. (Phys. Rev. A 53 (1996) 4021). Numerical
instabilities force us to use a layer-by-layer approach that has some broad
unexpected effects. Core correlation is found to be significant and therefore
requires to be described in an adequate orbital basis. The natural-orbital
basis provides an interesting alternative to the orbital basis from the
layer-by-layer approach, allowing us to overcome some deficits of the latter,
giving rise to magnetic dipole hyperfine structure constant values in excellent
agreement with observations. Effort is made to assess the reliability of the
natural-orbital bases and to illustrate their efficiency.
