LOCAL-DENSITY FUNCTIONAL CALCULATION OF THE 3-DIMENSIONAL BAND-STRUCTURE OF POLY(PHENYLENE VINYLENE) (PPV) AND COMPARISON WITH PHOTOEMISSION DATA

We have carried out a local-density-functional calculation of the thre e-dimensional (3D) band structure of the conducting polymer PPV in ord er to determine the effects of interchain coupling. The major effects are due to coupling of carbons with hydrogens on rings in adjacent cha ins. Among the effects of this coupling are: splitting by up to 0.4 eV at various points of the Brillouin Zone of the double degeneracy that would exist if the two chains in the unit cell were uncoupled; loss o f electron-hole symmetry; the wave vector at the valence band edge not along the chain direction; a band gap smaller by 0.2 eV than that obt ained from 1D calculations; and interchain coupling large enough to su ggest that polarons are unstable in PPV. The calculated density of sta tes is in reasonably good agreement with that found in photoemission.