[Ifeffit] EXAFS mean free path in small particles

[John J. Rehr]

   The question of the appropriate self-energy to be used in EXAFS
and XANES seems overdue for improvements on the conventional
Hedin-Lundqvist plasmon-pole approximation. Within Hedin's GW
approximation the self energy Sigma(E) is determined by poles
in G and poles in W = epsilon**(-1) V_coul. The electron gas
plasmon-pole approximation was derived for 3d metals, and is
questionable in insulators  and other systems such as small particles.
Indeed, the GW PP often overestimates loss.

  Efforts to improve on this are in progress, however, e.g., using
ab initio calculations of epsilon**(-1).  See for example,
``Electron self-energy calculation using a general
multi-pole approximation," J. A. Soininen, J. J. Rehr and E. L. Shirley,
J. Phys.: Condens. Matter, {\bf 15}, 2572 (2003).

   In the meantime, the self-energy can be adjusted in fits, e.g. to
the mean free path. I'd be interested in hearing experimental evidence
for the need for such corrections.

   J. Rehr