17th ETSF Workshop on Electronic Excitations: Advanced Green's function methods
- Myrta Gruning (Queen's University Belfast, United Kingdom)
- Matthieu Verstraete (Universite de Liege, Belgium)
- Claudio Attaccalite (Institut Neel, CNRS Grenoble, France)
- Nicole Helbig (University of Liege, Belgium)
- Fernando Nogueira (University of Coimbra, Portugal)
Since its first edition in 1995, the ETSF (former Nanoquanta) workshop has pioneered a new vision of theoretical electronic structure, through the appellation theoretical spectroscopy. The success of this framework, uniting several different communities of theorists around the quantities of interest to experimentalists, is by now patent. There is by now a community, far wider than the ETSF itself, which has adopted this banner. Several series of conferences, in particular under the auspices of CECAM, now cover different aspects of theoretical spectroscopy, its applications, and different simulation tools. Through it all the ETSF workshop has continued to be a precious opportunity for meeting and discussion within the theoretical spectroscopy community, providing systematic cross-fertilization with all major research groups in the field.The 2012 ETSF workshop will return to the fundamentals of theoretical spectroscopy, providing the in-depth coverage of Green’s function, perturbative, and non-perturbative theories which is necessary to push the field forward. It is thanks to this type of workshop that students receive solid and complete foundations in many-body and excited-state theory, and that experts have the time and opportunity to foster cutting-edge collaborations. ETSF 2012 will address many-body perturbation theory (GW, BSE), time-dependent density functional theory (TDDFT), quantum chemical methods and quantum transport (equilibrium and non-equilibrium). Experts both from within and outside the ETSF will be convened, and sessions will be followed by round-table format discussions, which have proven popular in past editions, and maximize exchange and collaboration.
Several families of first principles methods are available to study excited state properties of electronic systems. Well established approaches are based on Many-Body Perturbation theory, time-dependent density functional theory, quantum chemistry methods (coupled-cluster and Moeller Plesset perturbation theory), LDA+U and dynamical mean field theory. Such approaches give access to a wealth of quantities directly or indirectly linked to physical observables thus providing an important complement to experimental measurements.
TDDFT is routinely used for calculating vertical excitations (energies and strengths) of molecular systems, electron energy loss spectroscopy in semiconductors, and vibrational spectroscopy of both molecular and extended systems. Within the framework of many-body perturbation theory the GW approximation successfully describes quasiparticle excitations, as measured by direct and inverse photoelectron spectroscopy, and the Bethe-Salpeter equation provides accurate estimate for neutral excitations, giving access to e.g. optical absorption and electron energy loss spectra. Approaches based on coupled-cluster and Moeller Plesset perturbation theory are applied to small-medium molecular systems to provide even more accurate results for electronic excitations, often taken as benchmarks. Dynamical mean field theory  is mainly used to address direct and inverse photoemission spectroscopy in strongly correlated systems, that are problematic within the above-mentioned GW approximations. The GW and Bethe-Salpeter equation have been successfully applied to study electronic and optical properties of nanostructures, biological molecules, and novel materials (see e.g. [6,7,8]). Other successes of the strongly correlated theories are the magnetic structure of surface alloys  and the Kondo resonance in cerium .
Motivated by the great success of the field in recent decades, and stimulated by the flourishing of new spectroscopic techniques, experiments and applications, theorists are perfecting and extending the functionalities of the existing theoretical tools as well as developing novel approaches[11,12].
Attempts are being made to apply the different excited state theories to large systems (biomolecules, nanostructures, or interfaces, e.g. ). This implies using hybrid methods (e.g. QM/MM or DFT in DFT) to treat only part of the system with the full Many-Body arsenal, while preserving microscopic and quantum detail for the surrounding parts of the system, and/or the environment.
Novel algorithms have appeared to reduce the weight of calculations, in many cases with order-of-magnitude differences in computer time or storage space. Fundamental advances in non-equilibrium and transient-state theories (in particular with Kadanoff-Baym-based approaches e.g. ) are pushing the limit of calculations, and of what can be observed experimentally.
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