Comparative study of the effect of the Hubbard coefficient U on the properties of TiO2 and ZnO

First-principles calculations combined with Hubbard U correction based on Density Functional Theory has been used to investigate the effect of its inclusion on electronic, structural, and optical properties and to reproduce correct band gap for TiO2 (anatase and rutile) and ZnO (wurtzite) structures. The effect of the implementation of U for only metal (Ti or Zn) d state (Ud) or for both 3d (Ud) and Op (Up) states was investigated to exploring changes in valence and conduction bands and their origin. Bader's charge analysis has been used to revealing the nature of chemical bonding. We can conclude that for both TiO2 polymorphs as the Ud factor increases, both the BG and the lattice parameter increase. On the contrary, for ZnO, as Ud increases the value of BG increases but the cell parameter decreases. The optimal values correspond to Ud =8 eV (TiO2) and Ud =13 eV (ZnO). The corresponding optical properties using Ud, such as real and imaginary parts of dielectric function, reflectivity R(omega), refractive index n(omega), extinction coefficient k(omega), absorption coefficient alpha(omega) and electron energy loss function L(omega) has been calculated. It is important to highlight that the curves obtained considering the inclusion of the Ud show an excellent agreement with those reported experimentally.