Time evolution of our reference simulation. From left to right and top to bottom panels: the semi-major axis a_i, eccentricity e_i, period ratio of adjacent planets P_i+1∕P_i, the resonant angles corresponding to the 2:1 resonance of the outer and inner planet pairs (see e.g. Eqs.(8) and (9)) and the Laplace angle φ_L = λ_c − 3λ_b + 2λ_e. In the top left panel, dashed vertical lines mark the times at which Fig. 1 shows the surface density. The two dashed horizontal lines indicate the semi-major axis of the giant planets as observed.