Molecular dynamics simulation was carried, out to investigate the molecular motions of a wave-absorbing composite consisting of core/shell Fe-clusters/Al2O3 doped into a dimethyl-siloxane methyl vinyl polymer (Polysiloxane, Gum753). Analysis of various correlation functions showed resonant modes with frequencies of 15 GHz arising from tumbling of Fe clusters and large-amplitude synchronized torsion of polymer chains. The frequency of the motion depended on the length of the polymer and the Fe additive. The estimated frequency agreed well with the peak appearing in the experimental reflection loss spectra. The present study identified the origins of the spectroscopic observations. (C) 2017 Elsevier B.V. All rights reserved.