Distinct metallization and atomization transitions in dense liquid hydrogen

We perform molecular dynamics simulations driven by accurate Quantum Monte Carlo forces on dense liquid hydrogen. Recently it has been reported a complete atomization transition between a mixed-atomic liquid and a completely dissociated fluid in an almost unaccessible pressure range {[\emph{Nat. Commun.} {\bf 5}, 3487 (2014)]}. Here instead, in a much more interesting pressure range, we identify a different transition between the fully molecular liquid and the mixed-atomic fluid at $\sim$ 400 GPa, with numerical evidence supporting its metallic behavior. Therefore we predict that the metallization at finite temperature occurs in this partially dissociated molecular fluid, well before the complete atomization of the liquid. At high temperature this first-order transition becomes a crossover, in very good agreement with the experimental observation. Several systematic tests supporting the quality of our large scale calculations are also reported.