Departure coefficients b_i of selected Cr i and Cr ii levels as a function of optical depth at 500 nm. Panels a) to d) present results for the solar model atmosphere. a) The reference model atom with a total of 374 levels constructed as described in Sect. 2.1. Hydrogen collision rates are multiplied by S_H = 0.05. Photoionization cross-sections for Cr i are taken from Nahar (2009). b) Same as a), but hydrogenic photoionization cross-sections are used for all levels. c) Same as a), but S_H = 0. d) The reduced model atom with a total of 108 levels including the Cr ii ground state, S_H = 0.05. e) to h) same as a), but the model atmospheres are taken from the grid with stellar parameters specified in each panel. Note that some levels on the plots are in thermal equilibrium with the other levels.