Local density approximations (LDA) and generalized gradient approximations (GGA) to the density functional theory (DFT) are known to be insufficient to properly describe electronic structures of strongly correlated systems. A practical prescription to the falilure of the conventional functionals is to introduce on-site effective Coulombic interactions in the form of Hartree-Fock theory to LDA and GGA. We implemented the DFT+U method into the state-of-the-art linear combination of pseudo-atomic orbital (LCPAO) method based on the density functional theory [1]. By introducing a dual representation of the occupation number matrix instead of the on-site or full representations, the LDA+U formalism is refined to be consistent with a nonorthogonal LCPAO basis in regard to the sum rule of the total number of electrons. For typical transition metal oxide bulk systems, the band gap, magnetic moment, and detailed electronic structures are investigated with the different choices of basis orbitals and effective U values as well as the definition of the occupation number matrix. The results are in good agreement with previous theoretical and experimental studies, indicating that the proposed O(N) method is a quite promising approach for the study of large-scale correlated material systems consisting of localized electrons [2,3]. The research was conducted in collaboration with Prof. Yu's group in Seoul National University.

- "O(N) LDA+U electronic structure calculation method based on the nonorthogonal pseudoatomic orbital basis", M.J. Han, T. Ozaki, and J. Yu, Phys. Rev. B 73, 045110 (2006).
- "Anisotropic exchange interactions of spin-orbit-integrated states in Sr2IrO4", H. Jin, H. Jeong, T. Ozaki, and J. Yu, Phys. Rev. B 80, 075112 (2009).
- "Novel Jeff=1/2 Mott State Induced by Relativistic Spin-Orbit Coupling in Sr2IrO4", B. J. Kim, H. Jin, S. J. Moon, J.-Y. Kim, B.-G. Park, C. S. Leem, J. Yu, T. W. Noh, C. Kim, S.-J. Oh, J.-H. Park, V. Durairaj, G. Cao, and E. Rotenberg, Phys. Rev. Lett. 101, 076402 (2008).