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Chemistry plays an important role in the interstellar medium (ISM),regulating heating and cooling of the gas, and determining abundances ofmolecular species that trace gas properties in observations. Although solvingthe time-dependent equations is necessary for accurate abundances andtemperature in the dynamic ISM, a full chemical network is too computationallyexpensive to incorporate in numerical simulations. In this paper, we propose anew simplified chemical network for hydrogen and carbon chemistry in the atomicand molecular ISM. We compare results from our chemical network in detail withresults from a full photo-dissociation region (PDR) code, and also with theNelson & Langer (1999) (NL99) network previously adopted in the simulationliterature. We show that our chemical network gives similar results to the PDRcode in the equilibrium abundances of all species over a wide range ofdensities, temperature, and metallicities, whereas the NL99 network showssignificant disagreement. Applying our network in 1D models, we find that the$\mathrm{CO}$-dominated regime delimits the coldest gas and that thecorresponding temperature tracks the cosmic ray ionization rate in molecularclouds. We provide a simple fit for the locus of $\mathrm{CO}$ dominatedregions as a function of gas density and column. We also compare withobservations of diffuse and translucent clouds. We find that the $\mathrm{CO}$,$\mathrm{CHx}$ and $\mathrm{OHx}$ abundances are consistent with equilibriumpredictions for densities $n=100-1000~\mathrm{cm^{-3}}$, but the predictedequilibrium $\mathrm{C}$ abundance is higher than observations, signaling thepotential importance of non-equilibrium/dynamical effects.

A Simple and Accurate Network for Hydrogen and Carbon Chemistry in the ISM