Optical Properties of Fluid Hydrogen at the Transition to a Conducting State.
We have mapped the molecular-atomic transition in liquid hydrogen using first principles molecular dynamics. We predict that a molecular phase with short-range orientational order exists at pressures above 100 GPa. The presence of this ordering and the structure emerging near the dissociation transition provide an explanation for the sharpness of the molecular-atomic crossover and the concurrent pressure drop at high pressures. Our findings have nontrivial implications for simulations of hydrogen; previous equation of state data for the molecular liquid may require revision. Arguments for the possibility of a first order liquid-liquid transition are discussed.