Accuracy of metaGGA functionals in describing transition metal fluorides

Accurate predictions of material properties within the chemical space of transition metal fluorides (TMFs), using density functional theory (DFT) is important for advancing several technological applications. The state-of-the-art semi-local exchange-correlation functionals within DFT include the strongly constrained and appropriately normed (SCAN) and the restored regularized SCAN (r$^2$SCAN), both of which are meta generalized gradient approximation (metaGGA) functionals. Both SCAN and r$^2$SCAN are susceptible to self-interaction errors (SIEs) while modelling correlated $d$ electrons of transition metals. Hence, in this work, we evaluate the accuracy of both functionals in estimating properties of TMFs, including redox enthalpies, lattice geometries, on-site magnetic moments, and band gaps. We observe both SCAN and r$^2$SCAN to exhibit poor accuracy in estimating fluorination enthalpies among TMFs, attributable to SIEs among the $d$ electrons. Thus, we derive optimal Hubbard $U$ corrections for both functionals based on experimental fluorination enthalpies of binary TMFs. Note that the linear response theory yielded unphysical $U$ values for V, Fe, and Ni fluorides. While adding the optimal $U$ to the metaGGA functionals does not significantly affect the lattice volumes and magnetic moments, it does significantly increase the calculated band gaps. Also, we calculate the average Na intercalation voltage in Mn, Fe, Co, and Ni fluorides as a transferability check of our optimal $U$ values. Overall, we recommend using the Hubbard $U$ correction to improve predictions of redox enthalpies in other TMFs, while for band gap predictions, we suggest using the non-corrected functionals. Finally, our study should advance the accuracy of DFT-based screening studies to unearth novel TMFs, which can be used in various applications, including energy storage, catalysis, and magnetic devices.