The Workshop on Needs for a Photon Spectroscopy Theory Center was held on Saturday August 8, 1998 at the Advanced Photon Source at Argonne National Laboratory, i.e., just after the BSR Conference and just before XAFS X. The Workshop was sponsored by the DOE and Workshop organizers included J. J. Rehr (U. Washington), M. A. Van Hove (LBNL), P. A. Montano (ANL) and M. Newville (U. Chicago). This Workshop is a sequel to the ALS Workshop on Theory and Computation for Synchrotron Applications held in October, 1997 (see http://electron.lbl.gov/alsworkshop/ ).

The purposes of the Workshop were i) to summarize the needs for and ii) to develop a tentative plan for a Photon Spectroscopy Theory Center. The object of such a center Center is to enhance the scientific productivity of all synchrotron light sources through

1. advances in fundamental understanding of the interaction between photons and matter,
2. the development of efficient theoretical methods and computer codes for calculations of photon-matter interactions, and
3. the development of efficient scientific analysis tools that make use of theory to simulate and interpret experiments.

The Workshop format included a morning session of ten-minute invited talks, five sequential panel sessions in the afternoon on XAS Theory, DAFS and Other Spectroscopies, Photoelectron Diffraction and Holography, Basic Theory and Magnetism and Analysis Needs. Finally the various Workshop Conclusions were outlined in a Summary Session.

In addition to this Overview, extended abstracts of the presentations, a summary of the Workshop Findings , and links to related sites are available on the Workshop WWW page ( http://www.phys.washington.edu/~jjr/apsworkshop ). In brief, participants at the Workshop voiced unanimous support for the need for a Photon Spectroscopy Theory Center to utilize the major synchrotron facilities more effectively.

1. Unanimous support was voiced by Workshop participants for the need of a Photon Spectroscopy Theory Center in the Synchrotron Radiation research community that would serve both theorists and experimentalists. There was general consensus that such a Center devoted to theory, modeling and analysis would now be an appropriate sequel to the recent national investment in major photon sources, in order to use those sources most effectively. To paraphrase D. Koelling: There has been no comparable build up of theory during this period; although we are skilled on all fronts of theory, modeling and analysis at present, we are at the same time highly deficient on all of these fronts. Such a Center has the potential to revolutionize synchrotron radiation research, by developing efficient quantitative tools that could be used throughout the scientific community. Indeed this would lead to a paradigm shift by transforming photon spectroscopies to analytical tools. Given the ever increasing use of synchrotron facilities by thousands of scientists, the potential impact of such a Center for scientific research is enormous.

2. The invited workshop presentations discussed various topics that a Virtual Theory Center could address. Most speakers discussed the need for various improvements in theory or analysis tools. For example, many spectroscopies and techniques (e.g., XANES, inelastic scattering, photoelectron holography) still lack a quantitative theoretical basis, thereby limiting the useful information that can be derived from measurements. Others, like EXAFS, which have a solid quantitative basis, still lack user friendly graphical interfaces and on-line analysis tools. S. Bare emphasized the needs of industrial users of synchrotron facilities in particular for such efficient tools. For example, an efficient XANES tool would be of great technological interest in materials sciences since XANES is relatively easy to measure and contains both chemical and structural information. M. Newville emphasized the need for a theoretical toolkit with components (e.g., potentials, fitting codes etc) that could be tailored to new techniques. Although the main focus of this Workshop was on hard x-ray applications, a number of speakers discussed the important needs for various soft x-ray applications, such as ARPES and photoelectron diffraction.

3. The Panel and Summary Sessions identified a number of research topics as important for advances in interpreting synchrotron radiation experiments that could be addressed by a Theory Center: These include needs for:

   --Improved potentials and phase shifts for all spectroscopies.

   --Improved treatment of atomic motion and Debye Waller factors.

   --Improved theories of various spectroscopies, e.g., XANES, XMCD.

   --Improved treatments of many-body effects (self energies, magnetism, etc).

   --Improved and online data analysis tools for all spectroscopies.

   --User friendly graphical user interface for XAS, XAFS, XANES etc.

   --Modular theoretical tools e.g., an open source theoretical "toolkit."

   --The need to combine current total energy electronic structure and quantum chemistry codes with modern spectroscopy codes.

   --User help at synchrotrons for theoretical tools, software etc.

   --Summer schools and periodic workshops to spread new developments.

4. A tentative model structure for a distributed Theory Center "without walls" was discussed. It was felt important to combine the features of a virtual Center, connected globally via the internet, with some real facilities to permit more frequent person-to-person interactions. Thus a model Center should include at least two hubs (e.g. at Universities or National Laboratories) each with few long term Sr. Scientist positions, and some short term postdoc, grad student, and visiting positions. In addition a need was recognized for about one theoretical software specialist at each of the major synchrotron facilities. These specialists would be the equivalent of beamline scientists and would provide user support and local software development appropriate to a given facility. To maintain fluidity, the Center would focus on a few major topics of current interest each year, and update the focus periodically. In addition the Center would sponsor summer schools and extended workshops open to scientists throughout the synchrotron radiation community.