  Thermopower measurements of molecular junctions have recently gained interest
as a characterization technique that supplements the more traditional
conductance measurements. Here we investigate the electronic conductance and
thermopower of benzenediamine (BDA) and benzenedicarbonitrile (BDCN) connected
to gold electrodes using first-principles calculations. We find excellent
agreement with experiments for both molecules when exchange-correlation effects
are described by the many-body GW approximation. In contrast, results from
standard density functional theory (DFT) deviate from experiments by up to two
orders of magnitude. The failure of DFT is particularly pronounced for the
n-type BDCN junction due to the severe underestimation of the lowest unoccupied
molecular orbital (LUMO). The quality of the DFT results can be improved by
correcting the molecular energy levels for self-interaction errors and image
charge effects. Finally, we show that the conductance and thermopower of the
considered junctions are relatively insensitive to the metal-molecule bonding
geometry. Our results demonstrate that electronic and thermoelectric properties
of molecular junctions can be predicted from first-principles calculations when
exchange-correlation effects are taken properly into account.
