Sensitivity of the normalized alignment, ${\rho }_0^2(n_{\mathrm{H}},B=0)/{\rho }_0^2(n_{\mathrm{H}}=0,B=0)$, to uncertainties in the elastic O–H collisional operator D as a function of log₁₀(n_H/cm³). For each level, the solid curve is the reference calculation (D), while the shaded band spans the range obtained by uniformly scaling all elastic rates to D/10 and 10 × D. The gray band and black line refer to the S₂ level (3s⁵S, J = 2), and the blue band and line to the P₁ level (3p³P, J = 1) shown in Fig. 2. Results are for B = 0.