Ostriker-Peeble (OP) criterion: ratio of the total mean kinetic energy to the total random kinetic energy, calculated within the extent of the bar, (Π/T_mean(t; R < R_bar)), at t = 0 for the total (thin+thick) disc as a function of the maximum bar strength (S​_bar, max). Left panels: evolution for the rthickS models, and the middle and right panels: evolution for models rthickE and rthickG, respectively. The points are colour-coded by the corresponding thick-disc fraction, f_thick (see the colour bar). Different symbols represent thin+thick models with different scale heights. For details, see the text. The shaded region denotes the bar-stable zone according to the OP criterion.