Assessing exchange-correlation functional performance in the chalcogenide lacunar spinels GaM4Q8 (M=Mo, V, Nb, Ta; Q= S,Se) ASSESSING EXCHANGE-CORRELATION FUNCTIONAL ... WANG, PUGGIONI, and RONDINELLI

We perform systematic density functional theory (DFT) calculations to assess the performance of various exchange-correlation potentials Vxc in describing the chalcogenide GaM4Q8 lacunar spinels (M=Mo, V, Nb, Ta; Q=S, Se). We examine the dependency of crystal structure (in cubic and rhombohedral symmetries), electronic structure, magnetism, optical conductivity, and lattice dynamics in lacunar spinels at four different levels of Vxc: The local density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, and hybrid with fractional Fock exchange. We find that LDA underperforms the Perdew-Burke-Ernzerhof (PBE) and PBE revised for solids (PBEsol) GGA functionals in predicting lattice constants as well as reasonable electronic structures. The performance of LDA and GGAs can be improved both quantitatively and qualitatively by including an on-site Coulomb interaction (LDA/GGA+U) with a Hubbard U value ranging from 2 to 3 eV. We find that the PBE functional is able to produce a semiconducting state in the distorted polar R3m phase without on-site Coulomb interactions. The meta-GGA functional SCAN (strongly constrained and appropriately normed) predicts reasonable lattice constants and electronic structures; it exhibits behavior similar to the GGA+U functionals for small U values of 1 to 2 eV. The hybrid functional HSE06 is accurate in predicting the lattice constants but leads to a band gap greater than the experimental estimation of 0.2 eV in this family. All of the lacunar spinels in the cubic phase are metallic at these levels of band theory; however, the predicted valence bandwidths are extremely narrow (≈0.5eV). The DFT ground states of cubic vanadium chalcogenides are found to be highly spin polarized, which contrast with previous experimental results. With spin-orbit coupling (SOC) interactions and a Hubbard U value of 2 to 3 eV, we predict a semiconducting cubic phase in all compounds studied. SOC does not strongly impact the electronic structures of the symmetry-broken R3m phase. We also find that these Vxc potentials do not quantitatively agree with the available experimental optical conductivity on GaV4S8; nonetheless, the LDA and GGA functionals correctly reproduce its lattice dynamical modes. Our findings suggest that accurate qualitative and quantitative simulations of the lacunar spinel family with DFT requires careful attention to the nuances of the exchange-correlation functional and considered spin structures.