Programme

I.8 -- Invited, VCTP-49

Date: Wednesday, 31 July 2024

Time: 16:00 - 16:40

Koopmans-compliant functionals: a functional approach to spectroscopy

Nicola Colonna

Laboratory for Materials Simulations, Paul Scherrer Institut, 5232 Villigen, Switzerland

Spectral properties are essential to understand or engineer materials: from photoemission spectra (widely used in recent years to characterize all possible topological properties) to transport (for ICT technologies) to light absorption (for energy harvesting) or emission (quantum technologies). To date, the most common electronic-structure approaches to reliably compute these quantities are based on Green's function theory. Despite the continuous improvement, these methods are still limited in system size and complexity due to their computational cost and complexity. As an alternative to Green's function methods, Koopmans spectral functionals provide a novel and functional approach to simultaneously and accurately describe ground state properties and charged excitations of atoms, molecules, nanostructures and periodic crystals. This is achieved by augmenting standard density functionals with simple but physically motivated orbital-density-dependent corrections that enforce the correct description of the charged excitation process. After introducing the general framework and its implementation, I will present numerical simulations showing the excellent agreement with experiments and state-of-the-art Green's function methods in predicting ionization potentials of a large set of molecules (the GW100 test set), photoemission and absorption spectra of organic donors and acceptors, band gaps and band structures of semiconducting and insulating crystalline materials. Finally, I will discuss the connection with Green's function theory arguing that the state-dependent Koopmans potentials act as a local and orbital-dependent counterpart to the electronic self-energy, albeit including spin-dependent interactions and screening effects that are absent in standard diagrammatic approaches based on the random phase approximation (GW) and that would require the inclusion of self-screening and vertex corrections. Being this a functional framework, the straightforward advantages are that forces, and other derivatives are also readily accessible, and that the numerical parameters are those typical of density functional calculations.

Presenter: Colonna Nicola