This simple model predicts the centre of the filler peaks in Fig. remarkably well. On going from He to Xe, the ‖𝚽(X0, X0)‖ increase by an order of magnitude from 1.01 to 12.4 eV Å -2 , indicating that the larger filler atoms experience stronger restoring forces from the framework atoms. As per Eq. 5, a larger 𝑚 X and ‖𝚽‖ will respectively decrease and increase 𝑓 ̃X. From He to Ne, the ‖𝚽‖ increases by a factor of two but the mass increases fivefold, resulting in a ~30 % reduction in 𝑓 ̃X. On the other hand, from Ne to Ar the ‖𝚽‖ increases by ~3× while the mass doubles, producing an overall increase in 𝑓 ̃X. From Ar to Kr and Xe, the mass increases faster than the ‖𝚽‖ and the rattling frequency falls from 4.75 to 3.80 and 3.61 THz.