a factor of two) of the scale height of the Galactic disk, as well as the densities and total masses of both atomic and molecular Hydrogen, there are some important discrepancies: first, the molecular fraction near the mid plane is severely underestimated; second, the central density is almost a factor of 2 above the observed one; and third, the amount of diffuse ionized gas is extremely low over the whole radial range, especially in the inner regions.

The first two problems are closely related. One could easily specify a lower mid plane density, but then the amount of molecular Hydrogen would decrease to unacceptably low levels. It seems that, even after transmission through a column density of $10^{21}$ cm$^{-2}$, the intensity of the photodissociating continuum is still too high for molecules to form at the observed rate. The third problem is also related to the simple prescription adopted for the incident continuum, since almost all the ionizing radiation becomes absorbed before reaching the illuminated face. In reality, young stars are spread around the Galactic plane. Adding any thin source of ionizing photons at the mid plane results in an [Hii]{} region qualitatively similar to the one observed in Figure \[figN\]; 