Force multiplier M(t) as a function of the optical depth parameter t. The green line is obtained by calculating the line-force parameters from a line list database for a temperature of T = 62 kK and a gas density of ρ = 10^-11g cm⁻³. The red line is obtained by fitting this force multiplier with Eq. (11). The best-fit parameters for this particular temperature and gas density are , Q₀ = 689, and α = 0.70.