  We obtain in a direct and rigorous manner a transition from a stable
molecular hydrogen $nH_2$ single chain to the quasiatomic two-chain $2nH$
state. We devise an original method composed of an exact diagonalization in the
Fock space combined with an ab initio adjustment of the single-particle wave
function in the correlated state. In this approach the well-known problem of
double-counting the interparticle interaction does not arise at all. The
transition is strongly discontinuous, and appears even for relatively short
chains possible to tackle, $n=3\div6$. The signature of the transition as a
function of applied force is a discontinuous change of the equilibrium
intramolecular distance. The corresponding change of the Hubbard ratio $U/W$
reflects the Mott--Hubbard-transition aspect of the atomization. Universal
feature of the transition relation to the Mott criterion for the
insulator--metal transition is also noted. The role of the electron
correlations is thus shown to be of fundamental significance.
