The workshop is concerned with ab-initio calculations of excited
electronic states, and related spectroscopic properties, in condensed matter
applications as well as atomic and molecular physics (nanostructured materials).
In solid state physics, electronic excitations are normally studied
using a Green's-function formalism, based on self-energy operators. In
the 1960s the theory reached a relatively advanced stage for the homogeneous
electron gas [1], but, due to computational limitations, one had to wait
the mid-1980s for the theory starting to be systematically applied to realistic,
inhomogeneous systems [e.g. 2,3].
Since then there has been considerable activity in exploring applications
of the self-energy theory, notably at the level of the GW approximation,
and computational advances [e.g. 4-7] have made more complex applications
possible; see e.g. [8-10].
Major developments are now concerning three main directions: (i) more information than just the improvement of bandstructure should be inferred from GW calculations, like updated charge densities or total energies [11], or electron/hole lifetimes [12]; (ii) besides the one-electron addition and removal energies, which are often well described on the GW level, a realistic description of two-particle excitations is now starting to be feasible, but calculations are still very cumbersome [8,13-15]. (iii) For the absorption spectra of finite structures, calculations based on time-dependent density functional theory (TDDFT) in the adiabatic local density approximation (ALDA) have yielded promising results [16].
All those points are of great relevance to make connection with up-to-date characterizations by experimental spectroscopies: photoemission (single and two-photon, inverse and femtosecond-time resolved), photoabsorption, electron-energy-loss, X-ray absorption, scanning-tunneling-microscopy, near-field, etc..
Since work on all points (i)-(iii) demands for a considerable conceptual
and numerical effort, it is important that a continuous contact to other
scientific communities is established. In fact, similar approaches are
used in different contexts, and similar problems may be solved with different
methods. Past workshops of this serie have stimulated the discussion of
condensed matter physicists with theoretical chemists and nuclear
physicists, and have brought together the Green's functions community with
people using Time Dependent Density Functional approaches [17,18] (which
work with effective potentials rather than with explicit self-energies
(or their derivatives)). This link can be regarded as the natural counterpart
to the relation with theoretical chemistry: the latter are, roughly speaking,
designed for very high precision with corresponding numerical effort, whereas
DFT like approaches promise improved efficiency with, at least at present,
lower precision. On the other hand, the present knowledge on self-energy
calculations and new variational expressions for the many-body total energy
and response functions are very relevant for the development of new non-local
exchange-correlation functionals in time and space (see for example [19-21])
that are of key importance for the practical implementation of TDDFT approaches.
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