Establishing best practices to model the electronic structure of CuFeO2 from first principles

The cuprous delafossite, CuFeO2, has received significant attention in recent years as a potential photocathode material in photoelectrochemical water-splitting cells. Presented herein is an investigation of the electronic structure of CuFeO2 in the framework of density functional theory. We have benchmarked three of the most popular formulations for the treatment of the electron exchange and correlation interactions, highlighting their strengths and weaknesses in predicting electronic structures compatible with the available spectroscopic measurements. Although some features are correctly reproduced by the simplest approach, which is based on the generalized gradient approximation, this fails in describing the fundamental semiconducting character of the material. The introduction of the fully self-consistent Hubbard U correction in the exchange correlation functional accounts explicitly for the on-site Coulomb interaction among localized d electrons, thereby opening a gap in the band structure. However, our results indicate that the U correction disrupts the crystal-field splitting of the t(2g) and e(g) states, resulting in an inaccurate description of the conduction-band edge. We provide a qualitative and quantitative analysis to explain why the t(2g) and e(g) states behave differently when the Hubbard correction is switched on. We find that best practice for accurate, yet computationally viable, simulations of CFO makes use of hybrid functionals, where the fraction of exact exchange is not arbitrarily selected but tuned according to the static dielectric constant of the material. In this case, theoretical predictions are found to be in excellent agreement with experimental results.