Quantum Cafe: Lucia Reining

Title: Correlation and Collaboration: Strategies to design functionals

Abstract: The use of functionals of compact quantities, such as Green’s functions, the onebody reduced density matrix, or the density, is a powerful alternative to wavefunction-based methods. In this talk, we will see general concepts that are common to different approaches relying on functionals, briefly discussing motivations and ideas. We will investigate the difficulties of finding good approximations, and we will highlight strategies that allow us to consider Density Functional Theory, Many-Body Perturbation Theory, and Dynamical Mean Field Theory on the same footing. In particular, we will concentrate on the power of combinations: these can be combinations of methods, combinations of models and real materials, or both. Several examples will be given: we will show that wavefunction-based methods are crucial not only for benchmarks, but also to design ingredients for the functional approaches 1 . We will illustrate the power of inserting density functionals into the equations of many-body perturbation theory for enhanced speed and accuracy 2,3 . Finally, we will discuss how one may use models in a well defined and systematic way 4-6 , in order to design new classes of functionals. The discussion will be illustrated over a wide range of correlation strength with results for models and real materials.

September 30, 2025