The log of the H₂ fraction is shown as a function of the hydrogen density and (top panel) the cosmic ray and (bottom panel) the extra-heating rates. Gas is molecular in the lower right-hand corner where the density is high and the non-radiative heating rates are low. The gas is nearly fully molecular in the lower right-hand corner and becomes increasingly ionized along a diagonal running to the upper left-hand side. There are discontinuous jumps in the chemistry as the gas changes phases.