The 2:1 inclusion complex of p-tert-butylcalix[4]arene and p-xylene was investigated by a combination of inelastic neutron scattering, WS, experiments and MM3 molecular mechanics calculations. Since the INS spectrum depends only on the vibrational frequencies and the form of the normal modes, the reasonable agreement between experiments and calculations shows that they can be simulated by the MM3 model. Further it validates the use of MM3 to search for possible spectral signatures of a hypothetical 1:1 inclusion complex and for a partial study of the p-xylene dynamics within the cage. The latter is accomplished by locating the transition state of a complicated motion of the guest molecule inside the cavity. Transition-state theory is used to calculate the temperature-dependent rate constants, and subnanosecond time scales are obtained.