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 sp2-bonded conjugated polymers. However, the lattice part (H) 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. © 1990 The American Physical Society.