Three-dimensional structure and intrinsic defects in trans-polyacetylene.

We report first-principles local-density-functional pseudopotential calculations of static and dynamic electronic structure properties of crystalline 3D trans (CH-) " We .find a broken-symmetry ground state of P21/a symmetry with in-phase dimerizations. Using a Green's-function technique, we show that the 3D character of the electronic band-edge states strongly suppresses self-trapping, destabilizing polarons, and possibly bipolarons as well in perfectly ordered 3D trans (CH)-". Within strictly one-dimensional (1D) models of conducting polymers, such as trans-polyacetylene [(CH),l, one expects a variety of localized, "self-trapped, " non-linear excitations — kink solitons, polarons, and bipolar-ons — with possible significant effects on optical and transport properties. ' Present conducting polymers, on the other hand, are typically inherently three-dimensional (3D), partially crystalline films, in which interactions between polymer chains play an important role. ' Indeed, "solid-state screening" effects, both intrachain and interchain, are frequently invoked to explain the apparent differences between the excited states observed in trans (CH), and-those expected from extrapolations of both measurements and calculations on long but finite polyenes in the gas phase. Although several studies ' based on simplified theoretical models have suggested that 3D interchain coupling can have strong effects on the 1D excitations, previous ab initio theories of the 3D structure of both trans and cis-(C-H), have left for further investigation the critical question of whether the interchain couplings are sufficiently strong to destabilize the carrier self-trapping associated with intrinsic defects in 1D systems. In this Letter we attempt to answer this and related questions by reporting the results of extensive first-principles studies of 3D crystalline trans-(CH) " and intrinsic defects in this material. For the electronic structure of crystalline 3D trans-(CH) " , we employed parameter free local-density-functional theory (LDA) with ab initio norm-conserving pseudopotentials for carbon and the Coulomb potential for hydrogen in a momentum space representation. The exchange-correlation energy is evaluated with the functional given by Painter. ' The Kohn-Sham equations are solved in a plane-waves basis with up to 1400 plane waves corresponding to a maximum kinetic energy of 30 Ry. All total energies are converged to 1 meV. Since the crystal structure of 3D trans-(CH), in its ground state is still not firmly established, ' " we have determined it from self-consistent minimizations of the total energy within LDA, augmented by a calculation of the Hellman-Feynman forces on the ions. Experimentally , two possible structures have been proposed, "' namely the P21/n symmetry with an antiphase …