This paper summarizes the results of a molecular dynamics study of the clathrate silicate dodecasil 3C with di erent methane ® llings, which covered the in¯uence of cavities on the behaviour of entrapped molecules. The simulations treat all degrees of freedom of methane at room temperature. The dynamics of methane is in¯uenced very little by use of a¯exible silicate framework, as compared with use of a rigid one. The calculated infrared and Raman spectra show that two types of molecular environment can be clearly recognized from the vibrational behaviour of the molecules. Also, in the low-frequency region the rigid molecule motions are well di erentiated. In small cages the molecules are located near the centres of cages, whereas in large cages the molecules move along the walls of the cavities while their orientation is nearly free.