Consistent descriptions of metal–ligand bonds and spin-crossover in inorganic chemistry

Density functional theory (DFT) is today the unchallenged tool for routinely obtaining molecular information on chemical stability, reactivity, and electronic structure across the Periodic Table. The chemical bond is the fundamental unit of molecular structure and reactivity, and thus, large-scale DFT studies of inorganic systems in catalysis and bioinorganic chemistry rely directly on the ability to balance correlation effects in the involved bonds across the s-, p-, and d-blocks. This review concerns recent efforts to describe such bonds accurately and consistently across the s-, p-, and d-blocks. Physical effects and ingredients in functionals, their systematic errors, and approaches to deal with them are discussed, in order to identify broadly applicable methods for inorganic chemistry.

► A review is performed of systematic effects in theoretical inorganic chemistry.
► Entropy, exact exchange, zero-point energies, some relativistic effects, and self-interaction errors loosen electron densities, weaken bonds, and favor HS.
► Dispersion and some relativistic effects contribute to compact electron densities.
► Neglect of physical effects may cancel or enhance errors, leading to incorrect conclusions e.g. regarding the accuracy of density functionals.
► A “80-20 rule” of modeling is emphasized, arguing that these systematic effects should be addressed before elaborate new functionals are constructed.