HCN concentration in chemical equilibrium (with GGChem, Woitke & Helling 2003) as function of gas temperature for different CO₂ fractions in the gas mixture (other gases: 1% H₂, 1% H₂O, rest N₂). For T ≳ 3000 K (right dashed grey line), HCN concentration increases with increasing CO₂ concentration (until ~3–5% CO₂). For T ≲ 2700 K HCN concentrations decreases with increasing CO₂ concentration (except for 0.1% CO₂).