  We use 3D-PDR, a three-dimensional astrochemistry code for modeling
photodissociation regions (PDRs), to post-process hydrodynamic simulations of
turbulent, star-forming clouds. We focus on the transition from atomic to
molecular gas, with specific attention to the formation and distribution of H,
C+, C, H2 and CO. First, we demonstrate that the details of the cloud chemistry
and our conclusions are insensitive to the simulation spatial resolution, to
the resolution at the cloud edge, and to the ray angular resolution. We then
investigate the effect of geometry and simulation parameters on chemical
abundances and find weak dependence on cloud morphology as dictated by gravity
and turbulent Mach number. For a uniform external radiation field, we find
similar distributions to those derived using a one-dimensional PDR code.
However, we demonstrate that a three-dimensional treatment is necessary for a
spatially varying external field, and we caution against using one-dimensional
treatments for non-symmetric problems. We compare our results with the work of
Glover et al. (2010), who self-consistently followed the time evolution of
molecule formation in hydrodynamic simulations using a reduced chemical
network. In general, we find good agreement with this in situ approach for C
and CO abundances. However, the temperature and H2 abundances are discrepant in
the boundary regions (Av < 5), which is due to the different number of rays
used by the two approaches.
