Any spectroscopic technique perturbs the system under investigation and promotes it into an excited state. Experimentally the challenge then lies in the correct interpretation of the system's response. The challenge from a theoretical point of view, however, is to find (or develop) a suitable, accurate and most of all computationally tractable approach to describe the response of the system.

For electronic excitations three methods are now well established [1]: 

1.) Time-dependent density functional theory (TDDFT) is an elegant way to describe neutral excitations, either by propagation in real-time or within linear response theory. A current disadvantage is that the time-dependent exchange-correlation functional is not known exactly and common approximations often impose severe limitations on the accuracy of TDDFT, in particular for extended systems. At the same time the suitability of TDDFT for certain excitations [2], the description of quantum transport phenomena or the coupling to strong laser fields is actively being investigated.

2.) Many-body perturbation theory (MBPT):

The GW-approximation to the electronic self-energy has been very successful in the description of quasiparticle excitations as measured by direct and inverse photoemission (electron addition and removal energies). Adding the electron-hole interaction on the level of the Bethe-Salpeter equation provides acess to neutral excitations such as the optical spectrum. Some of the open issues concern self-consistency and the validity of the pseudopotential concept [3], as well as the improvement of the screening function inherent to both GW and BSE [4]. A theoretical and computational framework for non-linear optical phenomena is beginning to emerge [5]. Due to the unfavourable scaling with system size, that most current implementations suffer from, recent efforts have focused on the reduction of the computational complexity [6].

3.) Quantum chemical approaches: 

Moeller-Plesset perturbation theory or the coupled-cluster approach are hierarchical methods that provide increased accuracy with increasing order. Multiple excitations are easily included and due their accuracy couple-cluster calculations are often taken as benchmarks. However, the approaches are currently limited to small finite systems due to their computational cost, but faster algorithms and applications to solids are appearing [7]. Moreover, connections to many-body perturbation theory are being established [8].

These methods have now emerged as standard theoretical spectroscopy tools for e.g., optical absorption, electron-energy loss, angular resolved photoemission, core-hole spectroscopy, luminescence spectroscopy, etc.

The meeting will discuss recent advances in both conceptual developments as well as their application to realistic systems and realistic environments e.g. electron-phonon coupling for temperature effects [9] or inclusion of solvents [10]. Recent developments in using approaches from the realm of many-body perturbation theory or TDDFT to compute ground state total energies will also be covered. One prominent exmaple that has received increasing attention in the last few years due to the inclusion of long range van de Waals interactions is the random-phase approximation (RPA) [11,12].