Polaron versus bipolaron in conducting polymers: a density matrix   renormalization group study
  Competition between polaron and bipolaron inconjugated polymers with nondegenerate ground state is systematically studied in the extended Hubbard-Peierls model with the symmetry-breaking Brazovskii-Kirova term, using the density matrix renormalization group method combined with lattice optimization in the adiabatic approximation. We demonstrate that the relative stability of a bipolaron over two separated polarons sensitively depends on both on-site Hubbard $U$ and nearest-neighbor repulsion $V$. When $U$ is much larger than $V$, the bipolaron state is more stabilized compared with mean field calculations. 
Makoto Kuwabara, Yukihiro Shimoi, and Shuji Abe