Relationship between structure and conductivity in liquid carbon

We have performed extensive simulations of liquid carbon at various densities, using a highly transferable tight-binding model of carbon. In contrast to some recent theoretical suggestions, we show that the low-density liquid is primarily twofold bonded. Such a bonding arrangement leads to a low conductivity, compared to the primarily ${\mathit{sp}}^2$ bonded liquid that occurs at intermediate densities. This is in qualitative agreement with experiments that show that for pressures less than 100 kBar, the conductivity of liquid carbon increases with density. At high densities, the liquid is mainly tetrahedrally bonded, and the conductivity then decreases with increasing density.