  We explore the properties of the bipolaron in a 1D Holstein-Hubbard model
with dynamical quantum phonons. Using a recently developed variational method
combined with analytical strong coupling calculations, we compute correlation
functions, effective mass, bipolaron isotope effect and the phase diagram. The
two site bipolaron has a significantly reduced mass and isotope effect compared
to the on-site bipolaron, and is bound in the strong coupling regime up to
twice the Hubbard U naively expected. The model can be described in this regime
as an effective t-J-V model with nearest neighbor repulsion. These are the most
accurate bipolaron calculations to date.
