Vibrations and soliton dynamics of positively charged polyacetylene chains

Ab initio molecular dynamics simulation is performed on a small polyacetylene chain with a positive soliton defect. The dynamics is initialized by an external electric field. The collective motion of the carbon and hydrogen atoms are compared to some low frequency vibrational modes of positively charged polyacetylene chains of varying lengths having the characteristics of the soliton displacement. The soliton effective mass estimated using a variety of schemes is found to be about 10 electron masses. The static linear polarizability of singly charged polyacetylene chains of varying lengths is computed and compared with that of undoped chains. The electronic contributions to the polarizability are computed at the level of the coupled Hartree-Fock or the random phase approximation, and the vibrational contributions are estimated by invoking the double harmonic oscillator approximation. The soliton defect causes some enhancement of the electronic term, which covers 10-15 carbon-carbon double bonds, and it generates a large increase of the vibrational part, which in the absence of counter ions show no sign of leveling off even for the longest chains considered.