Doing the basics well: An assessment of meta-generalised-gradient density functional theories using chemically-diverse two-electron model systems

Meta-generalised-gradient density functionals are the most sophisticated density functional models routinely applicable to both isolated and periodic systems. Because benchmarking across both domains is challenging, we instead employ simple two-electron models that encompass bonding environments found in both molecules and materials. Highly parameterised functionals prove fragile outside their parameterisation domains, often failing to converge for delocalised bonds, whereas constraint-based functionals are generally more robust. Computational cost and fragility both increase with functional complexity, with notable exceptions of B97M-V (robust but expensive) and PKZB (fast but fragile). Overall, SCAN and MS functionals provide the best balance of cost, accuracy and robustness.