Beyond DFT Team @ THEOS


Members

Nicola Marzari (EPFL)
Iurii Timrov (EPFL)
Riccardo De Gennaro (EPFL)
Tommaso Chiarotti (EPFL)
Marco Vanzini (EPFL)
Alumni:
Matteo Cococcioni (University of Pavia)
Nicola Colonna (PSI)

Research:

  • Hubbard-corrected functionals (DFT+U and DFT+U+V): Predictive modeling of transition-metal and rare-earth compounds and of their physical properties is crucial for the development of many technologies including, for example, biomimetic photochemistry, catalysis, solar cells, accumulation devices as Li-, Na-, Mg-ion or Li-air batteries, recovery of waste heat through ferroelectricity, superconductivity, sensing and actuation, spintronics, multiferroics, quantum information. Unfortunately, first-principles calculations based on density-functional theory (DFT), almost an obligated choice to approach the study of systems of realistic complexity, suffer from the inability of most approximate energy functionals to capture ground states characterized by strongly localized and possibly correlated electrons. One of the corrective schemes that are used to alleviate these difficulties is based on the addition of Hubbard functionals acting on localized (atomic-like d or f) states, according to the so-called DFT+U scheme [1]. The most advanced approach, named DFT+U+V, is actually based on the extended Hubbard model with on-site and inter-site electronic interactions and allows to capture electronic localization even in presence of significant hybridization [2]. This extended corrective scheme is used in the study of a number of transition-metal compounds including materials for Li-ion batteries and complex oxides investigated for photo-catalysis. The group also develops and maintains a computational tool, based on density-functional perturbation theory (DFPT) [3], to compute the effective interaction parameters (U and V) from first-principles. The method of Ref. [3] has been implemented in the open-source Quantum ESPRESSO distribution and it is publicly available.
  • Koopmans-compliant spectral functionals: The interpretation of experimental spectra, such as those obtained with ultraviolet photoemission spectroscopy (UPS) or angular-resolved photoemission spectroscopy (ARPES), often requires theoretical support, due to the complexity of the data involved. From a theoretical point of view, photoemission spectra can be studied with many-body perturbation theory, density-matrix functional theory or with the wave function methods of quantum chemistry. However, due to the significant computational requirements of these approaches, and their own limits in terms of ultimate accuracy, applications are limited in system size and complexity. For these reasons simpler methods such as Hartree-Fock or ground-state DFT are still frequently employed to interpret photoemission spectra. However DFT is a ground state theory and there is obvious connection between the single particle energies of the auxiliary Kohn-Sham systems and the charged excitation of the real interacting systems. On the other side, Hatree-Fock eigenvalues do have a meaning of additional/removal energies, but miss important relaxation effects. Dabo and collaborators have introduced Koopmans-compliant (KC) functionals [4-6] to enforce a generalized criterion of piecewise linearity with respect to the fractional removal or addition of an electron from any orbital (and not only the HOMO) in approximate DFT functionals, and to extend to the entire electronic manifold the self-interaction linearization imposed by DFT + Hubbard U [1]. The condition of Koopmans' compliance is naturally akin to that of enforcing a correct description of charged excitations, and thus can lead to orbital energies that are comparable to the quasiparticle excitations of photoemission experiments [7-9]. Notably all this is achieved using just a functional formulation for spectral properties and thus bypassing completely more expensive and cumbersome diagrammatic techniques.

News

  • 01/06/2021: Check out our new article [16] that highlights the importance of inter-site Hubbard interactions in the beta phase of MnO2 using DFT+U+V.
  • 29/01/2021: Check out our new article [15] that describes how to compute Hubbard parameters self-consistently using density-functional perturbation theory in the ultrasoft and projector-augmented wave formulations.
  • 28/12/2020: Check out our new article [14] that describes how to compute Hubbard forces exactly when using orthogonalized atomic orbitals as a Hubbard manifold.
  • 19/08/2020: Our new article [13] outlines the crucial role of extended Hubbard functionals to describe the electronic structure of complex transition-metal oxides such as pristine and Ni-substituted LaFeO3, and paves the way to future studies on similar systems.
  • 09/07/2020: Our new article [12] about the self-consistent DFT+U+V study of oxygen vacancies in SrTiO3.
  • 19/02/2020: Our new article [11] on the Hubbard-corrected density functional perturbation theory with ultrasoft pseudopotentials was published today in Phys. Rev. B!
  • 08/03/2019: Check out our new article [10] on the application of the DFPT approach to calculate self-consistent site-dependent Hubbard U parameter in stoichiometric and defective SrMnO3. This work was done in collaboration within MARVEL with Dr. Chiara Ricca and Prof. Ulrich Aschauer (University of Bern).
  • 16/08/2018: Our new article [3] on the ab initio calculation of the effective Hubbard U parameter using density-functional perturbation theory was published today in Phys. Rev. B!
  • 23/05/2018: Our recent work on the extension of Koopmans' functional to periodic systems [7] appeared in Physical Review X!

Publications