  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.
