Combining DFT and many-body methods to understand correlated materials

Electronic and magnetic properties of strongly-correlated systems are typically controlled by a limited number of states, located near the Fermi level and well isolated from the rest of the spectrum. This opens a formal way for combining the first-principles methods of electronic structure calculations, based on the density-functional theory (DFT), with the model many-body techniques, formulated in a restricted Hilbert space of states near the Fermi level. The core of this project is the construction of “ab initio low-energy models'', which incorporate the physics of Coulomb correlations and provide a transparent picture for the low-energy properties. First, I will describe how such models can be constructed. The procedure consists of three major steps, starting from the electronic structure in the local-density approximation. (i) Construction of the kinetic-energy part using an exact version of the downfolding method; (ii) Construction of the Wannier functions; (iii) Calculation of screened Coulomb interactions using a hybrid approach, which combines the random phase approximation with the constraint DFT.