  Within the framework of nonequilibrium Green's functions, we investigate the
thermoelectric transport in a single molecular junction with electron-phonon
and electron-electron interactions. By transforming into a displaced phonon
basis, we are able to deal with these interactions non-perturbatively. Then, by
invoking the weak tunneling limit, we are able to calculate the
thermoelectricity. Results show that at low temperatures, resonances of the
thermoelectric figure of merit ZT occur around the sides of resonances of
electronic conductance but drops dramatically to zero at exactly these resonant
points. We find ZT can be enhanced by increasing electron-phonon coupling and
Coulomb repulsion, and an optimal enhancement is obtained when these two
interactions are competing. Our results indicate a great potential for
single-molecular-junctions as good thermoelectric devices over a wide range of
temperatures.
