Abstract
The low‐frequency (<20 meV) polarized vibrational density of states in crystalline polyacetylene has recently been derived from inelastic neutron scattering data [J. L. Sauvajol, D. Djurado, A. J. Dianoux, N. Theophilou, and J. E. Fischer, Phys. Rev. B 43, 14 305 (1991)]. The spectral profiles were found to be highly anisotropic and to vary considerably with the chain conformation (cis or trans). Here we investigate the characteristics of the low‐frequency vibrations using the molecular dynamics method. Densities of states are calculated from the simulations and directly compared with the data derived from the experiments. The anisotropic and conformation‐dependent profiles are mostly well reproduced. The simulation trajectories are further analyzed so as to determine the dynamical contributions to the observed spectra. The lowest‐frequency vibrations parallel to the chain axes, at 1.5 meV in the cis system and 4 meV in the trans system, are found to result from whole‐molecule rigid‐body translations, invariant with chain length. Conversely, the low‐frequency intramolecular vibrations are weakly dependent on chain conformation but vary strongly with the chain length.