Ab Initio Theoretical Spectroscopy

The interaction of matter with radiation (photons or electrons) is a fundamental experimental tool to study materials, both to understand their properties and to tune new functionalities for technological applications. Experimental spectroscopy, such as photoemission, optical absorption, X-ray and electron energy-loss, etc. probe electronic excitations. Density-functional theory (DFT), the standard model of condensed matter physics, can describe with good approximation the ground-state atomic structure, total energy, electronic density, phonon frequencies, etc. On the other hand, DFT cannot access electronic excitations and excited-state properties, one should go beyond DFT. In the last twenty years, other ab initio theories able to describe excitations and spectroscopy, have become available and more and more used: time-dependent density-functional theory (TDDFT) and many-body perturbation theory (MBPT) or Green function theory (GW approximation and Bethe-Salpeter equation BSE). Algorithms and codes have been developed to implement these theories and to provide computational solutions to calculate excitations, excited state properties and spectra. The latter have recently started to constitute a database for new paradigms such as high-throughput (HT) and machine learning (ML) techniques. Within this scientific framework, the present school aims at covering theoretical and numerical aspects of TDDFT and MBPT, up to HT and ML outcomes, resorting also to practical hands-on codes (ABINIT, DP, 2Light, EXC) implementing them.

Keywords:

• Electronic excitations and spectroscopy techniques: experiment vs theory;
• Time-dependent density-functional theory (TDDFT), exchange-correlation kernels;
• Ab initio Green functions, many-body perturbation theory (MBPT);
• GW self-energy, COHSEX, Bethe-Salpeter equation (BSE);
• Out-of-equilibrium and non-linear (under intense laser) spectroscopy.