H-F6 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{1.44072\,\lambda^2}{\lambda^2 - 0.0110481} + \frac{0.128999\,\lambda^2}{\lambda^2 - 0.0590812} + \frac{1.16854\,\lambda^2}{\lambda^2 - 115.022}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.04 \times 10^{-7} + 2(1.61 \times 10^{-8})\,\Delta T + 3(-6.01 \times 10^{-11})\,\Delta T^2 + \frac{7.65 \times 10^{-7} + 2(8.16 \times 10^{-10})\,\Delta T}{\lambda - 0.266}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$