Organisers

• Patrick Rinke (Fritz-Haber Institute of the Max-Planck Society, Berlin, Germany)
• Ludger WIRTZ (CNRS-IEMN and University of Lille, France)
• Valerio Olevano (CNRS Institut Neel, Grenoble, France)
• Gian-Marco Rignanese (Catholic University of Louvain, Belgium)
• Francesco Sottile (Ecole Polytechnique, Palaiseau, France)

Supports

German Research Foundation (DFG)

   CECAM

   Psi-k

Description

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]. 

References

[1] Giovanni Onida, Lucia Reining and Angel Rubio, Electronic excitations: density-functional versus many-body Green’s-function approaches, Rev. Mod. Phys. 74, 601 (2002)

[2] Andreas Heßelmann and Andreas Görling, Blindness of the Exact Density Response Function to Certain Types of Electronic Excitations: Implications for Time-Dependent Density-Functional Theory, Phys. Rev. Lett. 102, 233003 (2009)

[3] Ricardo Gómez-Abal, Xinzheng Li, Matthias Scheffler, and Claudia Ambrosch-Draxl, Influence of the Core-Valence Interaction and of the Pseudopotential Approximation on the Electron Self-Energy in Semiconductors, Phys. Rev. Lett. 101, 106404 (2008)

[4] M. Shishkin, M. Marsman, and G. Kresse, Accurate Quasiparticle Spectra from Self-Consistent GW Calculations with Vertex Corrections, Phys. Rev. Lett. 99, 246403 (2007)

[5] P. Romaniello, D. Sangalli, J. A. Berger, F. Sottile, L. G. Molinari, L. Reining, and G. Onida, "Double excitations in finite systems", Journal of Chemical Physics 130, 044108 (2009)

[6] Hugh F. Wilson, Deyu Lu, François Gygi, and Giulia Galli, Iterative calculations of dielectric eigenvalue spectra, Phys. Rev. B 79, 245106 (2009)

[7] Lorenzo Maschio, Denis Usvyat, Frederick R. Manby, Silvia Casassa, Cesare Pisani, and Martin Schütz, Fast local-MP2 method with density-fitting for crystals. I. Theory and algorithms, Phys. Rev. B 76, 075101 (2007)

[8] Gustavo E. Scuseria, Thomas M. Henderson, and Danny C. Sorensen, The ground state correlation energy of the random phase approximation from a ring coupled cluster doubles approach, J. Chem. Phys. 129, 231101 (2008)

[9] Andrea Marini, Ab Initio Finite-Temperature Excitons, Phys. Rev. Lett. 101, 106405 (2008)

[10] Adriano Mosca Conte, Emiliano Ippoliti, Rodolfo Del Sole, Paolo Carloni and Olivia Pulci, Many-Body Perturbation Theory Extended to the Quantum Mechanics/Molecular Mechanics Approach: Application to Indole in Water Solution, J. Chem. Theory Comput. 5, 1822, (2009)

[11] Deyu Lu, Yan Li, Dario Rocca, and Giulia Galli, Ab initio Calculation of van der Waals Bonded Molecular Crystals, Phys. Rev. Lett. 102, 206411 (2009)

[12] Huy-Viet Nguyen and Stefano de Gironcoli, Efficient calculation of exact exchange and RPA correlation energies in the adiabatic-connection fluctuation-dissipation theory, Phys. Rev. B 79, 205114 (2009)