If the filler atoms are modelled as moving within a rigid CoSb3 cage, an approximate rattling frequency 𝑓 ̃X can be determined by diagonalising a 3 × 3 dynamical matrix at 𝐪 = Γ built from the "self" force constants of the filler atom, i.e. with 𝑗 = 𝑗 ′ = X. For the cubic crystal this yields a single unique eigenvalue. Table lists for each filler atom the 𝑓 ̃X determined from this analysis together with the atomic masses 𝑚 X and the Frobenius norms of the 𝚽(X0, X0) force constants, calculated as: -Page 13 - Table Atomic masses 𝑚 X , force-constant norms ‖𝚽(X0, X0)‖ and approximate rattling frequencies 𝑓 ̃X of the filler atoms X in the five XCo8Sb24 models. The force-constant norms and 𝑓 ̃X are calculated using Eqs. 3-5.