Comparison between rate coefficients at T = 6000 K for collisional excitation of Na I by neutral hydrogen atoms. Only optically allowed transitions are shown and x-axis represents the energy of the transition. Top: the ratio between the unscaled Drawin formula and the free electron model of Kaulakys (1991) for transitions between highly excited states. Bottom: the ratio between the unscaled Drawin formula and the quantum mechanical calculations by Barklem et al. (2010) for transitions between levels up to 5s.