Ab initio self-consistent-field—linear combination of atomic orbitals—molecular-orbitals calculations on undoped and lithium-doped $p$-quaterphenyl, a model chain for poly-($p$-phenylene), demonstrate dramatic changes in geometrical and electronic structures upon doping. For a charge transfer of less than $0.1e$ per carbon atom, $p$-quaterphenyl becomes nearly coplanar and the rings adopt a strong quinoidal character, resulting in an important decrease of the band gap. This stresses the need for taking proper account of such modifications in order to comprehend the physics of the insulator-metal transition in the whole class of highly conducting organic polymers. Possible bipolaron defects in highly doped poly-($p$-phenylene) are shown to extend over about four rings.

• Received 28 June 1982

DOI:https://doi.org/10.1103/PhysRevB.26.6000