Bond-order-wave versus spin-density-wave dimerization in polyacetylene

The dimerization of polyacetylene is studied in the framework of a distance dependent Hubbard Hamiltonian for the valence pi electrons. The underlying one-dimensional many-body problem is solved numerically using the density matrix renormalization group (DMRG) method and the resulting ground-state energy is optimized in order to determine the dimerization delta and the average bond-length (R) over bar. The strength of the Coulomb repulsion U/t is varied from the uncorrelated or Huckel limit all over to the strongly correlated or Heisenberg case. While (R) over bar is not significantly affected by the value of U/t, delta shows a remarkable non-monotonic behavior. The differences between the bond-order-wave (small U/t) and the spin-Peierls; (large U/t) regimes are discussed.