Mg 3 Bi 2 : Recent work on Mg 3 Bi 2 -based materials have demonstrated their excellent room-temperature TE performance, which rivals the state-of-the-art Bi 2 Te 3 -based materials. We chose Mg 3 Bi 2 as the benchmark material to compare the TE performances of new materials considered in our search. The superior TE performance of Mg 3 Bi 2 is despite the fact that it is a semimetal with a measured band gap of -0.1 eV. Recall from the analysis in Section 2 that semimetals are generally at a disadvantage due to increased bipolar conduction. Based on our new descriptors, we find that Mg 3 Bi 2 is one of the best-performing room-temperature TE material (high n-type zT 300K ) among the 44 narrow-gap and semimetallic Zintl phases considered in this study (Figure ). The "rediscovery" of Mg 3 Bi 2 provides validation for our computational search workflow. Our calculated band gap (with HSE06+SOC) of -0.11 eV matches the measured gap very well. The computed TE quality factors ( , Table ) yield an R = 11, which is the highest among the 29 structures calculated with hybrid DFT functional. The lowtemperature TE performance of Mg 3 Bi 2 is often attributed to its semimetallic nature. Our two-band models shows that it is not the semimetallic nature but the large asymmetry parameter R that enables the high room-temperature TE performance. The large n and R is mainly a result of the high conduction band valley degeneracy N V,CB of 6 (Figure ), similar to Mg 3 Sb 2 . 61,62 SrSb 2 : This phase is predicted to have the highest maximum zT 300K among the 29 Zintl phases calculated with HSE06+SOC. SrSb 2 crystallizes in a monoclinic structure (space group P 2 1 /m, no. 11) composed of infinite 1-D Sb chains interspersed with Sr (Figure ). SrSb 2 has a narrow band gap of 0.32 eV and valence and conduction band valley degeneracy of 3 and 4, respectively (Table ). The Fermi surfaces are shown in 5c. The asymmetry parameter R (1.63) combined with a positive band gap (0.32 eV) results in a maximum n-type zT 300K that is 1.44⇥ compared to Mg 3 Bi 2 . SrSb 2 is isostructural with CaSb 2 and BaSb 2 -two other Zintl phases considered in this study, but predicted to have low maximum zT 300K . CaSb 2 has been experimentally shown to be a Dirac line-nodal material. While SrSb 2 is semiconducting (E g = 0.32 eV), CaSb 2 (-0.19 eV) and BaSb 2 (-0.36 eV) are predicted to be semimetallic. SrSb 2 has been used as a precursor for synthesizing Sr-filled CoSb 3 through arc-melting, and small grains of SrSb 2 have been observed in the samples. However, detailed measurements of the transport properties of SrSb 2 are not available and our prediction makes it an interesting candidate to pursue experimentally.