Achievability of inter-site effects within meta-GGA without fitted parameters

Determine whether improving the r2SCAN meta-generalized gradient approximation to accurately account for inter-site electron localization effects in non-compact covalent bonds can be achieved within a computationally efficient meta-GGA framework that uses both kinetic energy density and the Laplacian of electron density as ingredients and avoids material-dependent fitted parameters, or whether such improvements necessarily require functionals beyond the meta-GGA form.

Background

The paper identifies systematic failures of SCAN/r2SCAN for systems with non-compact covalent bonds (e.g., graphene, Cr2, VO2, Fe), attributing these to inadequate treatment of inter-site electron localization. The authors propose r2SCAN+V, a practical correction that improves accuracy across these cases by encouraging bond-centered electron localization.

Looking forward, the authors suggest that future revisions to r2SCAN should incorporate inter-site effects more effectively. They explicitly state uncertainty about whether such improvements can be accomplished within a meta-GGA that includes kinetic energy density and the Laplacian of the electron density while remaining non-empirical (i.e., without material-dependent fitted parameters), or if a beyond-meta-GGA approach is required.