  H2 is the simplest and the most abundant molecule in the ISM, and its
formation precedes the formation of other molecules. Understanding the
dynamical influence of the environment and the interplay between the thermal
processes related to the formation and destruction of H2 and the structure of
the cloud is mandatory to understand correctly the observations of H2. We
perform high resolution MHD colliding flow simulations with the AMR code RAMSES
in which the physics of H2 has been included. We compare the simulation results
with various observations including the column densities of excited rotational
levels. Due to a combination of thermal pressure, ram pressure and gravity, the
clouds produced at the converging point of HI streams are highly inhomogeneous.
H2 molecules quickly form in relatively dense clumps and spread into the
diffuse interclump gas. This in particular leads to the existence of
significant abundances of H2 in the diffuse and warm gas that lies in between
clumps. Simulations and observations show similar trends, specially for the
HI-to-H2 transition. The abundances of excited rotational levels, calculated at
equilibrium in the simulations are very similar to the observed abundances
inferred from FUSE results. This is a direct consequence of the presence of the
H2 enriched diffuse and warm gas. Our simulations show that H2 rapidly forms in
the dense clumps and, due to the complex structure of molecular clouds, quickly
spreads at lower densities. Consequently a significant fraction of warm H2
exists in the low density gas. This warm H2 leads to column densities of
excited rotational levels close to the observed ones likely revealing the
complex intermix between the warm and the cold gas in molecular clouds. This
suggests that the 2-phase structure of molecular clouds is an essential
ingredient to fully understand molecular hydrogen in these objects.

Astrophysics of Galaxies