  The Seebeck coefficient plays a fundamental role in identifying the
efficiency of a thermoelectric device. Its theoretical evaluation for atomistic
models is routinely based on Density Functional Theory calculations combined
with the Landauer-B\"uttiker approach to quantum transport. This combination,
however, suffers from serious drawbacks for devices in the Coulomb blockade
regime. We show how to cure the theory through a simple correction in terms of
the {\em temperature derivative} of the exchange correlation potential. Our
results compare well with both rate equations and experimental findings on
carbon nanotubes.
