Long-term evolution of HD 12661 system when varying the mutual inclination i_mut (x-axis) and the inclination of the orbital plane i (y-axis), both expressed in degrees. Left panel: maximal eccentricity of the inner planet, as defined by Eq. (8). Right panel: libration amplitude of the argument of the pericenter ω₁ (in degrees), as defined by Eq. (9). The three highlighted points are related to the representative planes shown in Fig. 3.