Anharmonic model for polyyne

With use of the first-principles, density-functional, full-potential, linear muffin-tin orbital (LMTO) method for helical polymers, calculations on carbon chains have been carried through. The results are used in providing parameters for model Hamiltonians related to the well-known Su-Schrieffer-Heeger model for ${\mathit{sp}}^2$-bonded conjugated polymers. However, the lattice part (${\mathit{H}}_{\mathrm{\sigma }}$) is found to contain strong anharmonic contributions. Using the models solitons, polarons, and ‘‘twistons’’ are examined. Due to the anharmonicity, polaronic states are energetically favored compared with solitonic states. Moreover, a rich variety of (meta)stable polarons is found. It is argued that the ground state of the charged system is a polaron lattice. Finally, twistons are found to be unstable when interchain couplings are neglected.