H-KF6 by CDGM

$$n(\lambda) = \sqrt{1 + \frac{0.0736996\,\lambda^2}{\lambda^2 - 0.0478549} + \frac{1.19277\,\lambda^2}{\lambda^2 - 0.00781133} + \frac{1.01749\,\lambda^2}{\lambda^2 - 104.242}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-5.94 \times 10^{-7} + 2(1.77 \times 10^{-8})\,\Delta T + 3(-1.13 \times 10^{-10})\,\Delta T^2 + \frac{5.53 \times 10^{-7} + 2(9.82 \times 10^{-10})\,\Delta T}{\lambda - 0.212}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$