Misrepresentation of thermal stability across different oxidation states of copper compounds by SCAN meta-GGA functionals.

The Strongly Constrained and Appropriately Normed (SCAN) meta-GGA (generalized gradient approximation) density functional and its regularized derivatives (e.g., SCAN and rSCAN) have been proposed as a post-standard exchange-correlation functional, and are widely believed to replace conventional GGA functionals (e.g. PBE-GGA) owing to greatly improved electronic structures of strongly correlated systems and overall accuracy of total energies. While these improvements have been widely demonstrated for various systems, we report a significant failure of SCAN functionals related to erroneous stability of multivalent states of copper: SCAN and its derivatives (rSCAN) critically fail to predict the relative stability of copper in oxidation states Cu(d) and Cu(d), excessively stabilizing Cu over Cu, which leads to wrong relative stability of CuO and CuO. This spurious bias also results in unphysical oxygen defect structures of YBaCuO for small δ. While the PBE-GGA functional can be fixed with a simple Hubbard-U correction (PBE + U) to predict both the spectral and thermochemical properties of copper compounds correctly, this is shown to not be the case for SCAN functionals. Our work advocates careful consideration of SCAN meta-GGA functionals when they are applied for cuprate superconductors, catalysis, and defect studies of copper compounds.