DFT Density Errors in Vanadium Dioxide Analyzed by Quantum Monte Carlo

Correlated materials such as VO2 are a challenge for current generation density functionals to model accurately. We use diffusion Monte Carlo to provide benchmark total energies and spin densities in VO2 which are fundamental to an accurate description of phase stability and magnetism. We evaluate local, semi-local, meta-GGA, hybrid, and Hubbard-corrected functionals against these benchmarks similar to Medvedev et al. [Science 355, 49 (2017)]. We also find that the best energetic description of the structural phases does not correspond to the most accurate density. However, we do find that an accurate spin density relates to a correct energetic ordering of magnetic states in VO2, though local, semilocal, and meta-GGA functionals partially demagnetize the V sites. The SCAN functional stands out as retaining a near balance of magnetization across the M1-R transition while correctly identifying the ground state crystal structure. In addition to ranking current density functionals, our reference energies and densities serve as important benchmarks for future functional development.