CECAM, Lyon, France

Institution: Ecole Polytechnique

Postal address: 91128 Palaiseau, France

Telephone: ++ 33 1 69 33 40 90

Fax: ++ 33 1 69 33 30 22

E-mail: albrecht@seurat.polytechnique.fr

Stefan Albrecht, Lucia Reining

Laboratoire des Solides Irradies, CNRS-CEA, Ecole Polytechnique,
F-91128 Palaiseau, France

Giovanni Onida, Rodolfo Del Sole

Istituto Nazionale per la Fisica della Materia, Dipartimento di
Fisica dell'Universita di Roma Tor Vergata, Via della Ricerca
Scientifica, I-00133 Roma, Italy

The description of the optical absorption process in solids often demands for the inclusion of excitonic effects beyond the one-particle picture. We treat such effects by solving an effective two-particle equation in a basis of Kohn-Sham eigenfunctions. This basis becomes very large, due to the large number of necessary k-points in the Brillouin zone sampling. We overcome this bottleneck by Fourier transforming to real space, thus using a basis given by pairs of bands -i.e. the possible products of a valence and a conduction band- and lattice vectors. We discuss the convergence of the various contributions, and their influence on the absorption spectrum at the example of silicon.

Institution: Institut fuer Festkoerpertheorie und Theoretische
Optik

Postal address: Max-Wien-Platz 1

D-07743 Jena

Germany

Telephone: 0049-3641-635902

Fax: 0049-3641-635182

E-mail: bechsted@ifto.physik.uni-jena.de

WWW home page: http://www.ifto.uni-jena.de

Friedhelm Bechstedt

Friedrich-Schiller-Universitaet, Max-Wien-Platz 1, D-07743 Jena,
Germany

In order to calculate the optical properties of semiconductors
the Bethe-Salpeter equation (BSE) for the polarization function
and the Dyson equation for the single-particle Green function
have to be solved simultaneously. We discuss such a solution including
dynamical screening. To consider the binding effects between electrons
and holes, i.e. the excitons, the inhomogeneous BSE is traced
back to a homogeneous one. It may be solved in k space by direct
diagonalization. First results are presented for silicon.

Institution: Dipartimento di Fisica dell'Universita' di Roma Tor
Vergata

Postal address: Via della Ricerca Scientifica,1 -00133 Roma,Italy

Telephone: +39 6 72594761

Fax: (39 6) 2023507

E-mail: CORRADINI@roma2.infn.it

M. Corradini, M. Palummo, G. Onida, R. Del Sole

Istituto Nazionale per la Fisica della Materia, Dipartimento di
Fisica

dell'Universita' di Roma Tor Vergata, Via della Ricerca Scientifica

n. 1 - 00133 Roma (Italy)

We present a fitting formula for the local field factor G(q) obtained
by a recent diffusion Monte Carlo calculation of Moroni et al./1/.
Our aim was to obtain both the exact asymptotic behaviour in the
small and large q regions for G(q), and a Fourier transform of
the exchange correlation factor K(q) in terms of analytical functions
(not depending by any additional parameter). Using Lorentzian
and Gaussian functions for fitting G(q), K(x) is expressed as
a simple Yukawa-like term, plus a Gaussian term.

/1/ S. Moroni, D. M. Ceperley, and G. Senatore, Phys. Rev. Lett.
75, 689, (1995).

Institution: Dipartimento di Fisica dell'Universita' di Roma Tor
Vergata

Postal address: Via della Ricerca Scientifica n. 1 - 00133 Roma
(Italy)

Telephone: +39/6/72 59 45 22

Fax: +39/6/20 23 507

E-mail: delsole@roma2.infn.it

Institution: Department of Physics, University of York

Postal Address: York YO1 5DD, U.K.

Telephone: +44 1904 432231

Fax: +44 1904 432214

E-mail: rwg3@york.ac.uk

WWW home page: http://www-users.york.ac.uk/~rwg3/

T. Pollehn** and R.W. Godby*

*Department of Physics, University of York, York YO1 5DD, U.K.

**Cavendish Laboratory, University of Cambridge, Cambridge CB3
0HE, U.K.

We present aproximate results for Hubbard systems for which exact
results are also calculated using exact diagonalisation. The quantities
calculated are the spectral function and total energy, and the
approximations used are *GW*, *GW* with a Hartree-Fock
vertex ("generalised *GW*"), and the *T*-matrix
approximation with and without exchange diagrams. We also investigate
the effect of self-consistency in the self-energy within the *GW*
approximation on the spectral function and total energy.

Institution: MPI-FKF

Postal address: Heisenbergstrasse 1, D-70569 Stuttgart, Germany

Telephone: +49-711-689 1593

Fax: +49-711-689 1595

E-mail: hedin@audrey.mpi-stuttgart.mpg.de

WWW home page: http://www/docs/frames.html

John Michiels(1), Lars Hedin(1) and John Inglesfield(2)

(1) MPI-FKF, Heisenbergstrasse 1, D-70569 Stuttgart, Germany

(2) Univ of Wales, College of Cardiff, PO Box 913, Cardiff CF2
3YB, GB

We compare different approximations to calculate core electron
photoemission, trying to understand the transition from the adiabatic
to the sudden limits. We study a model case of an atom embedded
in a semi-infinite jellium. At low photoelectron energies we use
the Inglesfield-Bardyszewski-Hedin quantum-mechanical approximation
(QM), at somewhat higher energies a semi-classical approach with
quantum-mechanically calculated loss from a photoelectron moving
on a trajectory (SC), and at high energies the Spicer three-step
model (TRANSP). Comparisons are also made with the Tougaard approach.
SC agrees quite well with QM already at say twice the plasmon
energy above threshold, while TRANSP does not agree until the
order of tenths of plasmons energies. The approach to the sudden
limit is illustrated by a curve over the integrated intensity
of the satellites relative to that of the main peak as a function
of photo-electron energy. For energies when the satellite electrons
have an energy below the plasmon threshold while the primary electron
has an energy above, this curve has a pronounced peak due to the
comparatively long mean free part of the electrons in the satellite.
As models for the dielectric function of the semi-infinite jellium,
we use the Inglesfield and the Bechstedt coupling functions, as
well as the RPA infinite-barrier function. The latter includes
interference between the incoming and reflected electrons. We
derive a general expression for the relation between an RPA-type
dielectric function and the fluctuation potential needed in the
QM approach. The Inglesfield, Bechstedt and RPA fluctuation potentials
show only minor differences, except at very low energies.

Ref. Bardyszewski W and Hedin L, 1985, Physica Scripta 32, 439-50

A new approach to the theory of photoemission in solids

Institution: Dipartimento di Fisica dell'Universita' di Roma Tor
Vergata

Postal address: Via della Ricerca Scientifica n. 1 - 00133 Roma
(Italy)

Telephone: +39/6/72 59 45 23

Fax: +39/6/20 23 507

E-mail: onida@roma2.infn.it

Institution: Tor Vergata University - Dep. of Physics

Postal address: Via della Ricerca Scientifica 1

Telephone: 06/7259-4-508/523

Fax: FAX 06/72594507

E-mail: palummo@axtov1.roma2.infn.it

Institution: Institut fuer Festkoerpertheorie und Theoretische
Optik

Postal address: Max-Wien-Platz 1,D-07743 Jena,Germany

Telephone: 0049-3641-636684

Fax: 0049-3641-635182

E-mail: pulci@ifto.physik.uni-jena.de

O. Pulci$^{1,2}$, R. Del Sole$^{1}$, G. Onida$^{1}$, F. Bechstedt$^{2}$, L.Reining$^{3}$

1 Istituto Nazionale per la Fisica della Materia - Dipartimento di Fisica dell' Universit\'a di Roma Tor Vergata -- Via della Ricerca Scientifica, I--00133 Roma, Italy;

2 Institut fuer Festkoepertheorie und Theoretische Optik, Friedrich--Schiller--Universitaet Max-Wien-Platz 1, 07743;

3 Ecole Polytechnique, 91128 Palaiseau, France

We have calculated the QP bandstructure of GaAs(110) in the GW approximation using the DFT-LDA eigenvalues and eigenfunctions as the starting point, and computing the QP energies in first--order perturbation theory by evaluating the diagonal components of $<\Sigma - V_{xc}>$. Some preliminary results for the non-diagonal components are presented.

The importance of the choice of the number and type of k-points, in which the screening function and the integrals appearing in the exchange end correlation part of the Self--Energy are calculated, is discussed in details. Our calculation of GW energies shows that filled states undergo different QP corrections, depending on their surface localization.

Keeping into account this result, we calculate the optical spectra of GaAs(110) with the inclusion of QP corrections, and discuss its changes with respect to the rigidly shifted LDA spectrum.

Institution: Ecole Polytechnique

Postal address: 91128 Palaiseau, France

Telephone: 0033-1-69333690

Fax: 0033-1-69333022

E-mail: reining@monet.polytechnique.fr

Lucia Reining, Laboratoire des Solides Irradie's, Ecole Polytechnique,

91128 Palaiseau, France, and Giovanni Onida, Dipartimento di Fisica,

Universita' di Roma "Tor Vergata", I-00133 Roma, Italy

We discuss a method for first-principles calculations of photoemission
spectra in small clusters, going well beyond a standard DFT-LDA
approach. Starting with a DFT-LDA calculation, we evaluate self-energy
contributions to the quasiparticle energies in the GW scheme.
The contributions of structural relaxations are taken into account.
We show the importance of these effects at the example of the
photoemission spectrum of SiH4. We also briefly discuss results
for longer hydrogenated silicon chains.

Institution: Departamento Fisica Teorica. University of Valladolid

Postal address: C/ Prado de la Magdalena s/n

Telephone: ++34-83-423263

Fax: ++34-83-423013

E-mail: arubio@mileto.fam.cie.uva.es

WWW home page: http://www.fam.cie.uva.es/~arubio

Institution: University of Cambridge

Postal address: Theory of Condensed Matter, Cavendish Laboratory,
Madingley Road, Cambridge CB3 0HE, United Kingdom

Telephone: +44 (0)1223 337340

Fax: +44 (0)1223 337356

E-mail: as10031@phy.cam.ac.uk

**Arno Schindlmayr**^{1} and R. W. Godby^{2}

^{1} *Cavendish Laboratory, University of Cambridge,
Madingley Road, Cambridge CB3 0HE, United Kingdom
*

**Abstract:
**Most

Institution: Department of Physics, University of York

Postal address: Heslington, York YO1 5DD, UK

Telephone: (+44) (0) 1904 432221

Fax: (+44) (0) 1904 432214

E-mail: ls9@york.ac.uk

L. Steinbeck, M.M. Rieger*, I.D. White*, and R.W. Godby

Department of Physics, University of York, Heslington, York YO1
5DD, UK

*Cavendish Laboratory, University of Cambridge, Cambridge CB3
0HE, UK

Some aspects of the recent technical development of the real-space
imaginary-time *GW* method are discussed. In the calculation
of the Green function, polarizability, and self energy in real
space these quantities are restricted to a "megacell"
(interaction cell) the shape and size of which must be consistent
with the k point grid used for the computation of the dielectric
matrix and the screened Coulomb interaction in reciprocal space.
The Coulomb interaction is cut off at the edge of the Wigner-Seitz
cell of the megalattice. Besides describing this megacell idea,
the treatment of the long-range part of the dielectric function
and the scaling of the computational time with system size are
briefly discussed.

Institution: Dept. of Theor. Phys., Lund University.

Postal address: Solvegat 14A, S-22362 Lund, Sweden.

Telephone: +46-46-2229069

Fax: +46-46-2224438

E-mail: barth@teorfys.lu.se

U. von Barth and R. van Leeuwen

Dept. of Theor. Phys.

Lund University, Sweden.

In the present work we have made an attempt to construct an accurate
approximation to the functional for the exchange-correlation energy
of density-functional theory. The approximation is based on modelling
the exact exchange-correlation hole by means of that of Hartree-Fock
theory multiplied by a correlation factor causing the hole to
become deeper and more localized. The hole can be made to obey
several exact relations such as a sum rule and a cusp condition.
Already, very crude versions of the correlation factor give an
accurate description of the true in hole in such diverse systems
as the H2 molecule, the He atom, and the electron gas. By adjusting
parameters in the correlation factor we can obtain accurate correlation
energies in these very different systems. Needless to say, the
excahnge energies are exact. The model is presently being tested
in a variety of systems.

Institution: Cavendish Laboratory

Postal address: Madingley Road, Cambridge CB3 OHE

Telephone: 01223 337460

Fax:

E-mail: idw1000@phy.cam.ac.uk

I. D. White(*) , R. W. Godby(+), M. M. Rieger(*), R. J. Needs(*)

*Cavendish Laboratory, University of Cambridge, Madingley Road,
Cambridge

CB3 OHE

+Department of Physics, University of York, Heslington, York Y01
5DD

Density-functional theory (DFT) performed within any local approximation
for exchange and correlation, such as the LDA, fails to reproduce
the correct asymptotic image form of the surface barrier experienced
by electrons. Using the recently developed space-time method for
many-body perturbation theory calculations, we calculate the non-local,
energy- dependent self-energy for electrons at an Al(111) surface
within the GW approximation.

The effect of the self-energy can be studied in terms of a state-dependent
local potential, which yields the form of the crossover between
the bulk and image potential. The relative contribution of exchange
and correlation to the image potential has been disputed in previous
theoretical studies. We find that, although the exchange hole
can give the asymptotic potential experienced by occupied state
electrons, this is not the case for unoccupied states, and that
a correct physical description always requires the inclusion of
correlation effects.

Institution: Commissariat a l'Energie Atomique

Postal address: CEA - CELV, F94195 VILLENEUVE SAINT GEORGES CEDEX

Telephone: (33) 1 45 95 64 72

Fax: (33) 1 43 86 74 10

E-mail: vast@limeil.cea.fr

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