Giovanni Onida, Lucia Reining
& Angel Rubio talk with ScienceWatch.com and
answer a few questions about this month's Emerging Research
Front Paper in the field of
The authors have also sent along images of their
Article: Electronic excitations: density-functional
versus many-body Green's-function approaches
G;Reining, L;Rubio, A
Journal: REV MOD PHYS, 74 (2): 601-659 APR 2002
Addresses: Univ Milan, Dept Phys, Ist Nazl Fis Nucl, Via
Celoria 16, I-20133 Milan, Italy.
Univ Roma Tor Vergata, Dipartimento Fis, Ist Nazl Fis Mat,
I-00133 Rome, Italy.
Ecole Polytech, CNRS, CEA,UMR 7642, Solides Irradies Lab,
F-91128 Palaiseau, France.
(addresses may have been truncated.)
Why do you think your paper is highly
This paper is focused on the description of excited state properties, by
further developing and uniting two different previously existing
theoretical approaches. Namely, we have established the bridge between
Many-Body Perturbation Theory (MBPT) and Time-Dependent Density Functional
Theory (TDDFT) to calculate the excited states, and fostered several
directions along the way leading to the routine performance of ab
initio computation of electronic excitations in physics, chemistry,
and biophysics, for academic or for industrial research purposes.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
Definitely, it describes a synthesis of knowledge and a new methodology. It
is not a common review compiling results and know-how: it provides new
insight and developments in a coherent and complete framework. This is one
of the important aspects behind the impact of the work, the novelty in
methodology and understanding. In 59 pages, this review spanned the
following subjects: after giving strong motivation toward theoretical
developments, which are required by new experiments, we introduce the
common ingredients to both Green's function theory (GFT) and TDDFT.
The role of the dielectric function, as well as that of local fields is
explained; then, 4-points and 2-points equations and kernels are
introduced. Green's functions, and their connection to experiments, are
briefly reviewed, prior to the introduction of Hedin's equations and the GW
method. The latter is compared with Density Functional Theory (DFT), for
the calculation of total energy differences. Density Functional Theory,
Then, we come to two-particle excited states and the electron-hole
interaction, leading to the Bethe Salpeter Equation (BSE). After that,
TDDFT is briefly reviewed, and extensively compared with the BSE scheme,
also giving some example in the limit of non-overlapping orbitals.
Finally, applications to infinite and finite systems, a list of FAQ, and
three appendices conclude the paper. The last appendix contains, in a
nutshell, the idea that it should be possible to derive, from the BSE
scheme, an exchange-correlation kernel to be used in TDDFT.
Would you summarize the significance of your paper in
The interaction between electromagnetic radiation and matter is of
fundamental interest. It creates excitations in the materials leading to
phenomena with enormous consequences in completely different domains such
as technology, chemistry, materials science, climate science, renewable
energies, biology, and medicine.
These consequences can be desired (like photosynthesis) or not (as in the
case of radiation damage due to nuclear waste), but are, in most cases,
quite complicated to describe and predict. This is not only true for the
systems one encounters in "real life," but also holds for the materials
that a scientist or engineer may investigate in an academic or industrial
More and more, spectroscopic techniques probing the electronic response are
used in the investigation of rather complex systems, in order to get
information about their electronic structure as well as their structural
properties or their chemical composition. Defects, impurities, or surface
reconstructions induce, in general, changes in the electronic states. These
modifications give rise to new structures, modified energetic positions, or
intensity changes in the electronic spectra. "Question matter, and the
electrons will answer!" so one could say, but, for this, one should
understand the electrons' language.
This explains why a large field of research in computational physics is
related to the characterization of the electronic, structural, and bonding
properties of many-electron systems: nanostructures, surfaces, and extended
solids as well as their interactions.
Our Review of Modern Physics article has become a key document in
the excited-state scientific community, and, with an increasing number of
citations, an inspiration for much further research. Moreover, versatile
numerical tools for treating electronic excitations, based on this
approach, have then been developed in our community. These tools are made
available to a wider scientific community through the
European Theoretical Spectroscopy Facility (ETSF).
How did you become involved in this research and were
any particular problems encountered along the way?
We realized that the communities of physicists and chemists, working with
two different methods—namely those using the Green's function
approach and those based on the time-dependent DFT—would have
benefited from a general formulation which could be applied to both.
One of the main problems was the lack of a common language. We found the
best way to encompass it by organizing a series of workshops, and inviting
people from both communities. Sharing formalism and establishing
similarities and differences, has in fact, finally become the key to
discovering new synergies from these two approaches.
Where do you see your research leading in the
The long-term impact of our work is expressed in the concept of the ETSF, a
platform offering to the scientific community, in both academia and
industry, a remote access to user-friendly spectroscopy computer codes (and
the associated expertise) for nanoscale systems and advanced materials.
ETSF has the ambition to become the world-wide reference point for the
quantum description of excitations in complex systems on the nanometer
scale. Groups that obviously benefit are large national and European
infrastructures like synchrotrons, companies working in fields like
materials design for opto-electronics, as well as smaller experimental and
theoretical groups in the field of nano-, bio-, and materials-science.
Do you foresee any social or political implications for
Not in the immediate future, but a potential social impact could stem from
applications of the ab initio methods for the prediction of electronic
excitations to diverse fields such as nanomedicine, renewable energy
(photovoltaic cells), and semiconductor nanostructured devices relevant for
Professor Giovanni Onida
Department of Physics
ETSF Core Node Team Coordinator
University of Milan
Research Director at CNRS (Centre National de la Recherche
Coordinator of LSI Theoretical Spectroscopy Group
Professor of Condensed Matter Physics
Vice-president for Scientific Development
European Theoretical Spectroscopy Facility (ETSF)
San Sebastian, Spain
Schematic representation of the excitations involved in spectroscopies such
as direct photoemission, inverse photoemission, and absorption. Probing
excitations allows one to gain insight on the structural and electronic
properties of matter..
Keywords: many-body perturbation theory, time-dependent density
functional theory, ab initio computation of electronic excitations,
physics, chemistry, biophysics, green's function theory, bethe salpeter
equation, electronic excitations, european theoretical spectroscopy
facility, nanomedicine, renewable energy (photovoltaic cells),
semiconductor nanostructured devices.