An ab initio approach to phonon spectrum of trans-polyacetylene

By an ab initio calculation based on the local density approximation of density-functional theory, the force constants up to the third nearest-neighbour carbon-hydrogen unit are determined to obtain the phonon spectrum of trans-polyacetylene for the first time. The calculated phonon frequencies at the Γ point coincide well with those of the observed infrared and Raman spectra of trans-polyacetylene. The motion of hydrogen atoms is shown to be important in determining the phonon spectrum of polyacetylene due to the contribution of the bending between the CH and CC bonds.