@NotAnotherName The criticality search function relies on root-finding functionality from scipy.optimize. Many of the methods for finding the root of a scalar function require that the first two guesses (based on the bracket you pass) have opposite signs. For a criticality search where you want keff=1, this really means “opposite sides of 1”, so your the low value of your bracketing interval should produce a keff < 1 and the high value of your bracketing interval should produce a keff > 1. Alternatively, you can specify the initial_guess parameter instead of bracket — this will cause it to use the secant root-finding method which doesn’t have any requirement on the sign.