Electronic properties of doped and defective NiO – perspectives from quantum Monte Carlo and density functional theory

NiO is a Mott (or charge-transfer) insulator and is difficult to describe using density functional theory (DFT). Doped Mott insulators such as NiO are of interest for various applications, but rigorous theoretical descriptions are lacking. Here, we use quantum Monte Carlo methods, which very accurately include electron-electron interactions, to examine energetics, charge- and spin-structures of NiO with various point defects, such as vacancies or substitutional doping with potassium. The formation energy of a potassium dopant is significantly lower than for a Ni vacancy, making potassium an attractive monovalent dopant for NiO. We compare our results with DFT results that include an on-site Hubbard U (DFT+U) to account for correlations and find relatively large discrepancies for defect formation energies as well as for charge and spin redistributions in the presence of point defects. This implies that schemes such as DFT+U are unlikely to capture responses that depend in subtle and complex ways on ground state properties such as charge and spin densities.