Molecular clouds are composed mainly of two molecules, H$_2$ and CO, and embedded in diffuse gas.
H$_2$ and CO form within the dense core regions of molecular clouds, whereas the embedding diffuse gas contains among other particles hydrogen as atomic hydrogen (H), and carbon as carbon ions (C$^+$).
From these, CO observations are easily accessible from the dense parts of the clouds, and further C$^+$ ions emit the bright [CII] line at $\lambda = 157$~$\mu$m, whereas H$_2$ is hard to observe. C$^+$ can be present also in molecular gas, and therefore one would like to estimate the mass of H$_2$ either from CO or [CII] observations.
We analyse the origin of the [CII] line emission with simulations. Within the SILCC-Zoom project the evolution of a molecular cloud is simulated with a spatial resolution of $\Delta x = 0.122$~pc. The simulation is performed by the state-of-the art code FLASH including self-gravity and simplified chemical network with heating, cooling, (self)-shielding, and the chemical evolution of hydrogen and carbon species. The cloud condenses out of the interstellar medium near a disc-plane of a spiral galaxy. We postprocess the simulation by the radiative transfer code RADMC-3D to investigate from which gas phase the [CII] line originates. We find that it is mainly the atomic gas which is bright in the [CII] line, suggesting that the [CII] line is not a suitable tracer for H$_2$, but traces the transitioning gas from H to H$_2$.