  Based on ab initio molecular dynamics simulations, we determined the melting
curve of magnesium (Mg) up to ~460 GPa using the solid-liquid coexistence
method. Between ~30 and 100 GPa, our melting curve is noticeably lower than
those from static experiments, but is in good agreement with recent shock
experiments. Up to ~450 GPa, our melting curve is generally consistent with the
melting points from first-principles calculations using the small-cell
coexistence method. We found that, at high pressures of a few hundred GPa, due
to the strong softening of interatomic interactions in the liquid phase,
solid-liquid coexistence simulations of Mg show some characteristics
distinctively different from other metal systems, such as aluminum. For
example, at a given volume, the pressure and temperature range for maintaining
a stable solid-liquid coexistence state can be very small. The strong softening
in the liquid phase also causes the unusual behavior of reentrant melting to
occur at very high pressures. The onset of reentrant melting is predicted at
~305 GPa, close to that at ~300 GPa from the small-cell coexistence method. We
show that the calculated melting points, considering reentrant melting, can be
excellently fitted to a low-order Kechin equation, thereby making it possible
for us to obtain a first-principles melting curve of Mg at pressures above 50
GPa for the first time. Similar characteristics in solid-liquid coexistence
simulations, as well as reentrant melting, are also expected for other systems
with strong softening in the liquid phase at high pressures.
