  Using density functional theory the atomic and electronic structure of sodium
are predicted to depart substantially from those expected of simple metals for
$r_s <$ 2.48 ($p > 130$ GPa). Newly-predicted phases include those with low
structural symmetry, semi-metallic electronic properties (including zero-gap
semiconducting limiting behavior), unconventional valence charge density
distributions, and even those that raise the possibility of superconductivity,
all at currently achievable pressures. Important differences emerge between
sodium and lithium at high densities, and these are attributable to
corresponding differences in their respective cores.
