Importance of van der Waals correction for energetics in transition metal monoxides

Density functional theory (DFT) is in principle exact for the ground-state properties of all materials, even for the strongly correlated materials such as the transition metal monoxides. However, DFT with various approximations to the exchange-correlation predict the zincblende or wurtzite structure ground state for MnO, instead of the experimental rocksalt phase. The correct ground-state phase has been obtained only by high-level random phase approximation and diffusion Monte Carlo (DMC). Here we propose and test for MnO, FeO, CoO, and NiO that a semilocal density functional can solve this problem by properly including both self-interaction and van der Waals corrections. The importance of the latter was previously unanticipated. The MnO structural energy difference from SCAN+rVV10+U agrees very well with that from DMC (SCAN is the Strongly Constrained and Appropriately Normed meta-GGA, rVV10 is the revised Vydrov-van Voohris nonlocal correlation functional, and the on-site U is determined from linear response).