A band gap of 0 eV means the material is metallic — it has no energy gap between occupied and unoccupied electronic states. This is a real physical result, not an error.
Common Cases
- Pure metals: Fe, Cu, Al, Au all have zero band gap
- Metallic compounds: Many transition metal borides, carbides, and nitrides are metallic
- Semimetals: Materials like Bi or graphite have zero or near-zero gaps with very low DOS at the Fermi level
- Heavily doped semiconductors: May appear metallic in DFT calculations
DFT Artifacts
In rare cases, GGA/PBE calculations may incorrectly predict a zero band gap for materials that are experimentally semiconducting. This happens most often for:
- Strongly correlated systems (rare earth compounds, Mott insulators)
- Materials where GGA+U corrections are needed but not applied
- Very small gap materials where the calculated gap falls below numerical noise
If you expect a non-zero gap but see 0 eV, check if the material has known strong correlation effects.