Electronic Structure and Magnetism of Correlated Systems: Beyond LDA

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The first-principle calculations of electronic structure, different ground state properties and excitation spectra of strongly correlated materials are very important in the modern microscopic theory of magnetism. The magnetic properties of parent high-temperature superconducting oxides, nondoped manganites and different ladder compounds are closely related to the electronic structure of these systems, which appeared to be the Mott insulators [1], due to strong electron-electron interactions. A standard way to investigate the excitation spectrum and magnetic properties of strongly correlated electronic systems is the model-Hamiltonian approach, such as the Hubbard model with several adjustable parameters [2,3]. The physics of the metal-insulator transition and properties of the Mott insulators are well studied in the one-band model with the strongly simplified bare electron spectrum, which is realized for the models with infinite lattice connectivity (d = ∞) [4,5]. To investigate strong interactions in real materials we have to develop a first principles approach that takes into account a complicated crystal structure with several atoms per unit cell, band degeneracy, and other important features of known correlated electron systems. Unfortunately, except for small molecules, it is impossible to solve many-body problem without severe approximations. The most successful first principles method for investigations of electronic structure and magnetism of weakly correlated materials is the density functional theory (DFT) within the local (spin) density approximation (L(S)DA) [6], where the many-body problem is mapped into a noninteracting system with an effective one-electron exchange-correlation potential that is approximated by that of the homogeneous electron gas. The LSDA has proven to be very efficient for many extended systems, such as large molecules and solids. However, it cannot describe adequately correlation effects such as the Mott transition, charge and orbital ordering, heavy fermion physics, and many others [1,3,5,7–10].