Frequency analysis of Jupiter’s longitude of pericenter in two simulations in which Jupiter and Saturn are locked in 2:1 MMR. Top: both gas giants have initial eccentricities of 0.025 and their orbits exhibit regular motion (simulation JSREG). Bottom: the gas giants’ initial eccentricities were set to 0.03 and their orbits are chaotic (simulation JSCHA). Each system was integrated numerically for 136 Myr, and to perform the Fourier analysis we have adopted an output time step of 2 years.