We perform ab initio calculations within the local density approximation for infinite, isolated chains of poly(para-phenylene) (PPP) and poly(para-phenylene-vinylene) (PPV). Phonon frequencies at $\overrightarrow{k}=\overrightarrow{0}$ and structural properties are investigated with special focus on the ring-torsion barriers. Our results for PPV indicate a planar geometry, while for PPP we find a ring-torsion potential that is not affected by next-nearest-neighbor rings. This suggests the existence of a multiply degenerate ground state for PPP, with chiral, ordered, or random angle-alternating configurations having the same energy. In addition, we couple these results to a simple molecular-dynamics simulation in order to investigate the finite temperature behavior of the systems.

• Received 14 January 2002

DOI:https://doi.org/10.1103/PhysRevB.67.205205