AN AB-INITIO APPROACH TO PHONON-SPECTRUM OF TRANSPOLYACETYLENE

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