Parameter dependence of the final water D/H ratio (surface+mantle) as a function of the cosmic-ray ionization rate (ζ) and external UV radiation-field strength (χ). The top and bottom panels correspond to cloud densities of n_cl = 10³ cm⁻³ and 10⁴ cm⁻³, respectively. From left to right, the metallicity decreases from Z = 1 to 0.2. The initial H₂ OPR and the parameter α are fixed at 10⁻² and 0.95, respectively, in all models. Red outlines indicate parameter sets with water D/H ratios within 30% of 0.01, and are therefore consistent with 3I/ATLAS within the adopted tolerance. Blue outlines indicate parameter sets with water D/H ratios between 5 × 10⁻⁴ and 3 × 10⁻³, consistent with those observed in nearby low-mass protostars. White regions indicate parameter sets for which the chemical network calculations did not converge.