  We present 3D "zoom-in" simulations of the formation of two molecular clouds
out of the galactic interstellar medium. We model the clouds - identified from
the SILCC simulations - with a resolution of up to 0.06 pc using adaptive mesh
refinement in combination with a chemical network to follow heating, cooling,
and the formation of H$_2$ and CO including (self-) shielding. The two clouds
are assembled within a few million years with mass growth rates of up to $\sim$
10$^{-2}$ M$_\mathrm{sun}$ yr$^{-1}$ and final masses of $\sim$ 50 000
M$_\mathrm{sun}$. A spatial resolution of $\lesssim$ 0.1 pc is required for
convergence with respect to the mass, velocity dispersion, and chemical
abundances of the clouds, although these properties also depend on the cloud
definition such as based on density thresholds, H$_2$ or CO mass fraction. To
avoid grid artefacts, the progressive increase of resolution has to occur
within the free-fall time of the densest structures (1 - 1.5 Myr) and $\gtrsim$
200 time steps should be spent on each refinement level before the resolution
is progressively increased further. This avoids the formation of spurious,
large-scale, rotating clumps from unresolved turbulent flows. While CO is a
good tracer for the evolution of dense gas with number densities $n \geq$ 300
cm$^{-3}$, H$_2$ is also found for $n \lesssim 30$ cm$^{-3}$ due to turbulent
mixing and becomes dominant at column densities around 30 - 50 M$_\mathrm{sun}$
pc$^{-2}$. The CO-to-H$_2$ ratio steadily increases within the first 2 Myr
whereas $X_\mathrm{CO}\simeq$ 1 - 4 $\times$ 10$^{20}$ cm$^{-2}$ (K km
s$^{-1}$)$^{-1}$ is approximately constant since the CO(1-0) line quickly
becomes optically thick.
