Abstract We compare observations of H i from the Very Large Array (VLA) and the Arecibo Observatory and observations of HCO⁺ from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the diffuse (A _V ≲ 1) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multiphase ISM simulations. Using a coarse grid of PDR models, we estimate the density, FUV radiation field, and cosmic-ray ionization rate (CRIR) for each structure identified in HCO⁺ and H i absorption. These structures fall into two categories. Structures with T _s < 40 K, mostly with N(HCO⁺) ≲ 10¹² cm⁻², are consistent with modest density, FUV radiation field, and CRIR models, typical of the diffuse molecular ISM. Structures with spin temperature T _s > 40 K, mostly with N(HCO⁺) ≳ 10¹² cm⁻², are consistent with high density, FUV radiation field, and CRIR models, characteristic of environments close to massive star formation. The latter are also found in directions with a significant fraction of thermally unstable H i. In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., nonequilibrium processes like turbulent dissipation and/or shocks) are required to explain the observed HCO⁺ in this direction. Similarly, while our observations and simulations of the turbulent, multiphase ISM agree that HCO⁺ formation occurs along sight lines with N(H I) ≳ 10²¹ cm⁻², the simulated data fail to explain HCO⁺ column densities ≳ few × 10¹² cm⁻². Because a majority of our sight lines with HCO⁺ had such high column densities, this likely indicates that nonequilibrium chemistry is important for these lines of sight.