-- Application of modern excited state
electronic structure theory to x-ray spectroscopy, optical response, etc.
Development of shared computer codes that implement theory. These codes can be
used to analyze and preduct electron and optical excitations, based on recent
advances in excited state electronic-structure theory. This development willl
enable better utilization of data from synchrotron and other major DOE
facilities to probe the behavior of complex materials. The initial focus will
be on x-ray spectra, as calculated by multiple-scattering theory combined with
GW and TDLDA methods.
J. Rehr and E. Shirley: Team EPA leaders
J. Rehr X-ray spectroscopy theory – development of ab initio
codes for XAS spectroscopy calculations (XAFS, XANES, XMCD, etc.). Close
interactions with users at the ALS, APS, SSRL, NSLS, and other DOE facilities;
implementation of theories developed by teams EEC and TDP through modular code
developments and collaborations with other members (Albers, Shirley, Martin,
Levine, and Van Hove). Recent developments include modularization of the FEFF
codes for various spectroscopies and implementation of fast, parallel
computation versions based on the MPI protocol in collaboration with C. Bouldin
and J. Sims at NIST. Tests carried out, e.g., at NERSC on the IBM SP3 massively
parallel machine, have verified speedups of nearly two orders of magnitude.
E. Shirley X-ray Absorption and Scattering -- Investigation of
electron/optical excitations in x-ray and UV/VIS/IR regions, including core and
valence excitations and IR phonons, applied to electron spectroscopy, index
dispersion, absorption, fluorescence and scattering in collaboration with Rehr,
Benedict and others. Problems that have been identified for work include
realistic treatment of electron life-time effects and the intercomparison of
multiple-scattering- and pseudopotential/Bethe-Salpeter-based near-edge x-ray
absorption calculations(collaboration between E. Shirley and J. Rehr).
R.C. Albers: Ab initio electronic structure methods and
applications to XAS; extension to PES and XPES of the multiple-scattering
theory previously successfully applied to XAS; ultimate goal is a tractable,
quantitative theory and associated codes (analogous to FEFF) for PES, including
GW selfenergy effects and many-body excitations; collaboration with other
members (Rehr, Wilkins, VanHove).
J. Wilkins: Optical response due to real material structure;
hyper molecular dynamics for nano- or defected-material provide structures for
optical response computations with GW/BSE or TDLDA.
Z. Levine: TDLDA, Applications to x-ray spectroscopy and
optical response.
M. VanHove: Generalization of current methods of electron
diffraction at low energies to provide sensitivity to electronic, magnetic and
similar valence-level effects; interaction with team EPA members and users at
the ALS. Recent work has focused on non-spherical potentials and the electron
hole. With the long-range goal of treating valence-electron emission and low
final-state kinetic energies, we have implemented methods to include the
effects on multiple scattering of non-spherical atomic scattering potentials
and the electron hole left behind by the emitted electron. The new methods have
been first implemented and tested for the case of the CO molecule, which
represents a severe test case, due to its strongly localized bonding and its
strong resonance which provides enhanced sensitivity to the electronic and
bonding structure of the molecule. The agreement between theory and experiment
is very encouraging.
D. Saldin: Theories of the self-energy excited state electrons
in an inhomogeneous electron gas for a better modelling of X-ray and electron
spectoscopies, e.g., electron energy loss spectroscopy (EELS), X-ray absorption
spectroscopy (XAS), and very low energy electron diffraction (vLEED) for
applications such as the low energy electron microscope (LEEM), and the point
projection electron microscope (PPEM).
V. Andropov: magneto-optical response theory; LDA+U theory.
-- Development of theories of electronic
excitations and computer codes to implement theory.
S.
Louie and L. Benedict : Team EEC
leaders
S.
Louie : ; electronic excitations
using many-body Green's function approaches such as the GW approximation for
quasiparticle properties and BSE approach for optical properties; collaboration
with other members (Benedict, Canning, Chou and Northrup) on methods and applications
to complex materials; collaboration with teams EPA and TDP (Chelikowsky,
Martin, Shirley and Pantelides).
L. Benedict: Calculations (together with E. Shirley) of optical
absorption of materials under pressure using BSE methods; development (with S.
Louie) of method for band-gap renormalization in laser-excited semiconductors.
M. Y. Chou: Optical response of hydrides of rare-earths and
their alloys that exhibit switchable optical proper ties using the GW
approximation and beyond, in collaboration with S. Louie and others.
R. M. Martin: Improving density functionals for excited states.
Collaboration with Chelikowsky on TDDFT and real space grid methods; with Louie
on many body theory; and with Stechel on efficient response functions.
Development (with Wilkins, Rehr, Canning, and others) of modular codes with
well-defined interfaces that permit code sharing and development of separate
modules.
J. Northrup: Industrial applications; study of photoemission and
optical responses of semiconductor surfaces; collaboration with Louie, Chou,
and others.
-- Development of time-dependent approach
to excited states and response functions, and computer codes to implement
theory.
J. Chelikowsky and S. Pantelides: Team TDP leaders
J.
Chelikowsky We have been
investigating implementations of the time dependent local density approximation
(TDLDA) for computing electronic excitations in clusters and other confined
systems, e.g., quantum dots. Our present goals are three fold: (1) To develop
efficient and accurate methods for predicting the excitation spectra of matter
based on density functional theory. (2) To understand dielectric and optical
spectra of confined systems. (3) To compare and contrast TDLDA with other
methods. Some of our current projects include: Determining the full, nonlocal
dielectric matrix for bare clusters, predicting the optical excitations in
quantum dots such as passivated silicon quantum dots, comparing real time treatments
of TDLDA with frequency domain methods, and contrasting TDLDA with GW and
Bethe-Salpeter methods. We have written codes to solve the Kohn-Sham equations
in real space and solve the TDLDA problem in the frequency domain. These codes
will be made available on at our web site. We have been interacting with groups
at the University of Washington (George Bertsch). the University of California
at Berkeley (Steve Louie), and the University of Illinois (Richard Martin).
Serdar Ogut is a postdoctoral fellow in the group being supported by the CMSN
program. References: I. Vasiliev, S. Ogut, and J.R. Chelikowsky, Phys. Rev.
Lett. 78, 4805 (1997); S. Ogut, S.G. Louie and J.R. Chelikowsky, Phys. Rev.
Lett. 79, 1770 (1997); I. Vasiliev, S. Ogut, and J.R. Chelikowsky, Phys. Rev.
Lett. 82, 1919 (1999).
S. Pantelides: Time dependent response, linear and non-linear
optical response; direct time integration of TDDFT equations; interaction with
Chelikowsky, Louie and others on the time-dependent exchange-correlation
potentials beyond the adiabatic approximation. The project aims at full
implementation of time-dependent density functional theory in the nonlinear
regime to study interactions of molecules and solids with intense light. We
already have a first round in which the time-dependent evolution of the
electron system with frozen nuclei has been implemented and tested in atoms,
molecules, and crystalline Si with interesting results. In the next stage, we
plan to include simultaneous ionic motions in order to get the full nonlinear
response.
S. Leung: Development of large-scale GW/improved TDLDA/OEP
computational algorithms, in collaboration with Stechel and Quong and
consultation with Louie, Chelikowsky, Martin, and others. We have been working
on optical properties of strained III-V semiconductors and alloys using density
functional theory plus quasiparticle corrections. These include the large band
gap nitrides as well as arsenide-antimonide compounds. We are also currently
using simple models to study the many-body physics associated with the accurate
calculation of optical properties.
R. L. Martin: Improving Accurate calculations of molecular
excitation energies and polarizabilities are essential for modeling
spectrospcopic probes, addressing structure-function relations and screening
species for desired optical properties. We have been exploring the
applicability the random phase approximation (RPA) of time-dependent
Hartree-Fock theory to describe excited states in conjugated organic molecules
and oligomers. The RPA, when used with the semi-empirical Hamiltonians
developed previously to treat the ground states of these systems, gives very
good agreement with experimental excitation energies, oscillator strengths, and
nonlinear optical properties in those series examined thus far. We are
currently investigating the applicability of this approach to address geometry
changes accompanying excitation.
E. Stechel: Linear response within TDDFT, especially in
semiconductors and disordered solids; collaboration with team C members (Leung,
Martin, and Quong) on theoretical and methodological developments and
applications and connections to team B approaches.
A. Quong: TD, TEM, EELS; collaboration with Stechel and Leung
on linear response methods.
G. Bertsch: Numerical TDLDA calculations of interest to other
members of the group, e.g. amorphous carbon, with Stechel and Chelikowsky.
A. Canning: Scientific computing; development of excited state
algorithms and codes for parallel platforms; interaction with members on
computational issues.
W. McCurdy: Scientific computing resources; excited states of
molecular systems.