DFT(+U+SOC) Investigation of Na₂IrO₃

We report on DFT(+U+SOC) calculations of Na₂IrO₃. We find that miniscule perturbations to the crystal structure often lead to significantly different relaxed energies and band gaps – corresponding to different combinations of Ir 5d orbitals. The tendency of DFT+U to get trapped in such local energy minima suggests an explanation for discrepancies in the literature. Comparing ground state solutions with and without U and SOC shows that inclusion of SOC is necessary and sufficient to stabilize the experimental zigzag order and to open a band gap, casting doubt on the theory that Na₂IrO₃ is a Mott insulator. Investigating the band structures yields support for the quasi-molecular orbital theory over the J_eff = 1/2 model.