THE POLARIZED DENSITY-OF-STATES OF CRYSTALLINE POLYACETYLENE - MOLECULAR-DYNAMICS ANALYSIS AND COMPARISON WITH NEUTRON-SCATTERING RESULTS

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.