Atomic data necessary for the non-LTE analysis of stellar spectra

The determination of physical parameters and chemical abundances of stars based on non-local thermodynamic equilibrium (non-LTE) line formation requires a large amount of accurate atomic data on energy levels, photoionization cross-sections, oscillator strengths, inelastic collision cross-sections, and related data. In this paper, we briefly review astrophysical problems where non-LTE modelling leads to a better interpretation of observed data due to the use of advanced atomic data and where atomic data are still missing or uncertain. For the statistical equilibrium of atoms in cool stars, a top priority problem is the efficiency of inelastic collisions with hydrogen atoms.