Dear all, I’m writing because I’ve been trying to analyze some thermal dependent EXAFS data for the first shell of a c-Ge sample and ended up in a conceptual confusion. If any of you could give me some help on this I’d be very grateful. Whenever we fit an EXAFS spectrum with Artemis/IFEFFIT, we get a /delR/ value. As far as my (very) limited comprehension goes, /delR/ is the difference between the theoretical (FEFF8) bond length, /R0/, and the result of the best fit to my data, /R/, which gives the experimental bond length. So, this should be the /first cumulant/ of my distance distribution, right? The net linear thermal expansion /A/ (as defined by Anatoly Frenkel and John Rehr in [PRB48, 585, 1993]) from an one-dimensional model seems to have been considered as equal to the temperature variation of the /delR/ values by several people, including Matt Newville (on his thesis). Following this approach, it is possible to relate the EXAFS cumulants (/delR/, /ss/, /C3/ and /C4/) of the distance distribution with the constants of an one-dimensional anharmonic effective potential, following a correlated Einstein model. I was going through this path... … when I came across some papers (P. Fornasini and G. Dalba in [PRL82, 4240, 1999; PRB70, 174301, 2004] and E. A. Stern in [J.Phys.IV 7, C2-137,1997]) where an issue is raised; it is said that the thermal evolution of the first cumulant /C1/* of the real distance distribution in a crystal (which was said to be equivalent to /delR/) is NOT equal to /A/, because in a crystal there are vibrations perpendicular to the bond direction which are not considered in the one-dimensional model. It is argued that the temperature dependence of these vibrations perpendicular to the bond direction give raise to a positive shift of the minimum of the effective pair potential, while the net thermal expansion /A/ accounts only for changes due to the asymmetry of the potential. So, the main questions hammering my head are: - does /delR/ include any contribution from vibrations perpendicular to the bond direction? Can really /delR/ and /C1/* be the same quantity? - is the thermal evolution of /delR/ equal to the net thermal expansion /A/? Or should some correction for perpendicular vibrations be added to relate both quantities? Sorry if I made it too long, but it was something hard to express in a few words… Any help will be greatly appreciated. Cheers, Leandro