Comparisons of the time evolution of the central H₂ number density for models used in the interpolation. The black lines represent simulations using a full chemical network, and the red lines represent simulations using our approximate method. Different line styles are used to denote models with different physical parameters (see legend). The green line represents a model with the same physical parameters as the fiducial model, which also includes the effects of grains. Different physical conditions have a more drastic effect on the evolution of the cloud than the inclusion of grains.