Chemical models for HCO⁺. Left panel: HCO⁺ abundance versus the local total H-nucleus number density, n_H, for different CR ionization rates. In all cases, the abundance of HCO⁺ peaks for densities 8 × 10³ ≲ n_H ≲ 3 × 10⁵ cm⁻³. Right panel: Temperature profile, with the iso-(ζ/n) lines overplotted in orange, for different CR ionization rates (ζ_CR = 10⁻¹⁶ − 10⁻¹² s⁻¹, with the same color code as the left panel).