  We study the influence of the choice of cooling function on the formation of
molecular clouds in high-resolution three-dimensional simulations of converging
flows. We directly compare the results obtained using the simple, parametrized
cooling function introduced by Koyama & Inutsuka (2002) and used by a number of
converging flow studies with the results of the detailed calculation of the
non-equilibrium chemistry and thermal balance of the gas. We find that a number
of the cloud properties, such as the mass and volume filling fractions of cold
gas, are relatively insensitive to the choice of cooling function. On the other
hand, the cloud morphology and the large-scale velocity distribution of the gas
do strongly depend on the cooling function. We show that the differences that
we see can largely be explained by differences in the way that Lyman-alpha
cooling is treated in the two complementary approaches, and that a proper
non-equilibrium treatment of the ionisation and recombination of the gas is
necessary in order to model the high-temperature cooling correctly.
  We also investigate the properties of the dense clumps formed within the
cloud. In agreement with previous models, we find that the majority of these
clumps are not self-gravitating, suggesting that some form of large-scale
collapse of the cloud may be required in order to produce gravitationally
unstable clumps and hence stars. Overall, the physical properties of the dense
clumps are similar in both simulations, suggesting that they do not depend
strongly on the choice of cooling function. However, we do find a systematic
difference of around 10K in the mean temperatures of the clumps produced by the
two models.
