  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.
