  Heavily doped semiconductors are by far the most studied class of materials
for thermoelectric applications in the past several decades. They have Seebeck
coefficient values which are 2-3 orders of magnitude higher than metals, making
them attractive for thermoelectric applications. Semimetals generally
demonstrate smaller Seebeck coefficient values due to lack of an energy
bandgap. However, when there is a large asymmetry in their electron and hole
effective masses, semimetals could have large Seebeck coefficient values. In
this work, we study the band structure of a class of 18 semimetals using first
principles calculations and calculate their Seebeck coefficient using the
linearized Boltzmann equation within the constant relaxation time
approximation. We conclude, despite the absence of the band gap, that some
semimetals are good thermoelectrics with Seebeck coefficient values reaching up
to 200 $\mu$V/K. We analyze the metrics often used to describe thermoelectric
properties of materials, and show that the effective mass ratio is a key
parameter resulting in high Seebeck coefficient values in semimetals.
