  First principles molecular dynamics simulations reveal a liquid-liquid phase
transition in supercooled elemental silicon. Two phases coexist below
$T_c\approx 1232K$. The low density phase is nearly tetra-coordinated, with a
pseudogap at the Fermi surface, while the high density phase is more highly
coordinated and metallic in nature. The transition is observed through the
formation of van der Waals loops in pressure-volume isotherms below $T_c$.
