Self-interaction Correction Approach Research Articles

Improved band gaps and structural properties from Wannier–Fermi–Löwdin self-interaction corrections for periodic systems

The accurate prediction of band gaps and structural properties in periodic systems continues to be one of the central goals of electronic structure theory. However, band gaps obtained from popular exchange–correlation (XC) functionals (such as LDA and PBE) are severely underestimated partly due to the spurious self-interaction error (SIE) inherent to these functionals. In this work, we present a new formulation and implementation of Wannier function-derived Fermi–Löwdin (WFL) orbitals for correcting the SIE in periodic systems. Since our approach utilizes a variational minimization of the self-interaction energy with respect to the Wannier charge centers (WCC), it is computationally more efficient than the HSE hybrid functional and other self-interaction corrections that require a large number of transformation matrix elements. Calculations on several (17 in total) prototypical molecular solids, semiconductors, and wide-bandgap materials show that our WFL self-interaction correction approach gives better band gaps and bulk moduli compared to semilocal functionals, largely due to the partial removal of self-interaction errors.

Fractional occupation numbers and self-interaction correction-scaling methods with the Fermi-Löwdin orbital self-interaction correction approach.

We present a new assessment of the Fermi-Löwdin orbital self-interaction correction (FLO-SIC) approach with an emphasis on its performance for predicting energies as a function of fractional occupation numbers (FONs) for various multielectron systems. Our approach is implemented in the massively parallelized NWChem quantum chemistry software package and has been benchmarked on the prediction of total energies, atomization energies, and ionization potentials of small molecules and relatively large aromatic systems. Within our study, we also derive an alternate expression for the FLO-SIC energy gradient expressed in terms of gradients of the Fermi-orbital eigenvalues and revisit how the FLO-SIC methodology can be seen as a constrained unitary transformation of the canonical Kohn-Sham orbitals. Finally, we conclude with calculations of energies as a function of FONs using various SIC-scaling methods to test the limits of the FLO-SIC formalism on a variety of multielectron systems. We find that these relatively simple scaling methods do improve the prediction of total energies of atomic systems as well as enhance the accuracy of energies as a function of FONs for other multielectron chemical species.

Half-metallic ferromagnetism in ordered LaBaCo2O6 and disordered La0.5Ba0.5CoO3: DFT+U study

Ab-initio calculations are performed to investigate the magnetic structure of ordered LaBaCo2O6 and disordered La0.5Ba0.5CoO3 systems using density functional theory (DFT). Structural relaxation by atomic force minimization and volume optimization are taken into account for both compounds. In confirmation with experimental data, ferromagnetic ordering is found energetically more favorable as compared to nonmagnetic and antiferromagnetic systems. For electronic properties, the calculations have been extended to treat the Co 3d states by self-interaction correction approach. We show that sufficiently high value of U yields the majority spin and minority spin channels, metallic and semiconductor respectively, making the systems half-metallic. The magnitude of Co magnetic moment is higher in ferromagnetic ordering than antiferromagnetic case, while a significant amount of moment is found on the O atoms. The iso-surface plots of ordered and disordered system exhibit that the charge on the Co atom is localized.

Exchange coupling in transition metal monoxides: Electronic structure calculations

An ab initio study of magnetic-exchange interactions in antiferromagnetic and strongly correlated $3d$ transition metal monoxides is presented. Their electronic structure is calculated using the local self-interaction correction approach, implemented within the Korringa-Kohn-Rostoker band-structure method, which is based on multiple scattering theory. The Heisenberg exchange constants are evaluated with the magnetic force theorem. Based on these the corresponding N\'eel temperatures ${T}_{N}$ and spin-wave dispersions are calculated. The N\'eel temperatures are obtained using mean-field approximation, random-phase approximation and Monte Carlo simulations. The pressure dependence of ${T}_{N}$ is investigated using exchange constants calculated for different lattice constants. All the calculated results are compared to experimental data.

Polarizability of molecular chains: A self-interaction correction approach

Standard density functional approximations greatly overestimate the static polarizability of long-chain polymers, but Hartree-Fock or exact exchange calculations do not. We show that simple self-interaction corrected approximations afford a viable alternative for accurate polarizability calculations within density functional theory.

The Hartree product and the description of local and global quantities in atomic systems: A study within Kohn–Sham theory

The Hartree product is analyzed in the context of Kohn–Sham theory. The differential equations that emerge from this theory are solved with the optimized effective potential using the Krieger, Li, and Iafrate approximation, in order to get a local potential as required by the ordinary Kohn–Sham procedure. Because the diagonal terms of the exact exchange energy are included in Hartree theory, it is self-interaction free and the exchange potential has the proper asymptotic behavior. We have examined the impact of this correct asymptotic behavior on local and global properties using this simple model to approximate the exchange energy. Local quantities, such as the exchange potential and the average local electrostatic potential are used to examine whether the shell structure in an atom is revealed by this theory. Global quantities, such as the highest occupied orbital energy (related to the ionization potential) and the exchange energy are also calculated. These quantities are contrasted with those obtained from calculations with the local density approximation, the generalized gradient approximation, and the self-interaction correction approach proposed by Perdew and Zunger. We conclude that the main characteristics in an atomic system are preserved with the Hartree theory. In particular, the behavior of the exchange potential obtained in this theory is similar to those obtained within other Kohn–Sham approximations.

Localized and band states in superconducting oxides: La 2CuO 4

The problem of electron state localization in La 2 CuO 4 was investigated by the self-interaction correction approach (SIC) using the LMTO-Green function method. Localization was found not only for the fully occupied d xy , d xz and d yz copper states, but also for the d x 2 − y 2 orbitals which give the main contribution to the half-filled conduction band. The localized wave function corresponding to the d x 2 − y 2 orbital has only 57% of the charge density on a copper atom. The rest is distributed between the oxygen atoms in the plane due to the strong pdσ-hybridization.