  A first-principles approach is presented for the thermoelectricity in
molecular junctions formed by a single molecule contact. The study investigates
the Seebeck coefficient considering the source-drain electrodes with distinct
temperatures and chemical potentials in a three-terminal geometry junction. We
compare the Seebeck coefficient in the amino-substituted and unsubstituted
butanethiol junction and observe interesting thermoelectric properties in the
amino-substituted junction. Due to the novel states around the Fermi levels
introduced by the amino-substitution, the Seebeck coefficient could be easily
modulated by using gate voltages and biases. When the temperature in one of the
electrodes is fixed, the Seebeck coefficient varies significantly with the
temperature in the other electrode, and such dependence could be modulated by
varying the gate voltages. As the biases increase, richer features in the
Seebeck coefficient are observed, which are closely related to the transmission
functions in the vicinity of the left and right Fermi levels.
