Revisiting DFT+U calculations of TiO2 and the effect of the local-projection size

The method of density functional theory (DFT) combined with Hubbard U correction has been employed in wide-ranging applications in correlated systems. Although the DFT+U method can dramatically improve the treatment of many strongly correlated systems, careful attention must be paid to those parameters that significantly influence results. By varying the local-projection size with the muffin-tin radius, we examine the effect of the Hubbard U correction on lattice parameters, electronic structure, and the relative stability of TiO2 between rutile and anatase. Our results show that different ranges of the local projection can produce strikingly different quantitative results, leading to distinct conclusions. The quantitative discrepancies are attributed to the sensitivity of the occupancy numbers for the correlated orbitals toward the size of the projection subspace.