Mutual-neutralization reduced rate coefficients for the temperature T = 6000 K as a function of the electronic bound energy E_f. The bound energy is measured from the atomic ionization limit. The solid line shows the reduced rate coefficient calculated by means of the present simplified model; the symbols show the coefficients calculated by means of both the full quantum approach and the model approach for different collision partners and for different final states (hence with different electronic bound energies): H⁻ + Li⁺ (Croft et al. 1999a,b), Na⁺ (Barklem et al. 2010), Mg⁺ (Guitou et al. 2015), Al⁺ (Belyaev 2013b), Be⁺ (Yakovleva et al. 2016), Ca⁺ (Belyaev et al. 2016).