The performance of hybrid density functionals in solid state chemistry

We examine the performance of hybrid (HF-DFT) exchange functionals within Density Functional Theory (DFT) in describing the properties of crystalline solids. Recent applications are reviewed, and an extensive set of new results presented on transition metal compounds. The features of the electronic density and of several calculated properties are examined as the weight of the HF (exact) exchange in the hybrid functional is increased. Clear trends emerge in the structural and electronic properties; in particular, HF exchange increases the electronic localisation. This features causes an increase in the ionicity of the materials, a systematic decrease of the lattice parameter and increase of the elastic constants and bulk moduli. When HF and standard (LDA or GGA) DFT solutions yield systematically errors with opposite sign with respect to experiment, the formulation of hybrid functionals improves the accuracy of the calculations. This is the case for band gaps, phonon spectra, magnetic coupling constants, and all properties that depend on the extent of electronic localisation at either perfect or defective lattice sites. A different weight of HF exchange is required to reproduce the experimental value of different observables; as a general rule, however, a higher fraction of HF exchange than the 20% optimised in the molecular B3LYP formulation is required in the solid state.