Ab initio simulation of the metal/nonmetal transition in expanded fluid mercury

We present an investigation of the variation of the structural and electronic properties of liquid mercury for states along the liquid-vapor coexistence line, spanning the range from the triple point to the critical point. Our study is based on ab initio density-functional molecular dynamics on the Born-Oppenheimer surface. Of central interest is the metal/nonmetal transition occurring at densities approximately twice the critical densities. We show that the density-functional calculations describe the atomic structure very accurately over the entire range from the triple point to the critical point. We find that a single-particle gap between the 6s and the 6p band opens at a density of about 8.8 g ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, i.e., very close to the density where optical measurements locate the onset of the formation of an optical gap. The detailed investigation of the band edges and of the participation ratio of the eigenstates suggests that the metal/nonmetal transition is best described as a simple band-crossing transition and that electron localization and many-body effects may not be as important as assumed in current scenarios for the transition.