On Tuesday 31 August 2004 10:06 am, Kelly, Shelly D. wrote:
-Most often negative values is a strong indication that something in the model is not quite right. Be sure to look at your data and fits showing the individual contribution from each path in the real or imaginary part of the FT.
I wanted to expand on this little part of Shelly's excellent response to Timm's question. The ability to quickly and easily plot the data, the fit, and each individual path is one of Artemis' main strengths. You should definately do as Shelly suggests and compare to the data each path that you consider including in the fit. This comparison should be made as the real and/or imaginary parts of chi(R) as well as the magnitude. And this comparison should be done both before and after a fit. Sometimes problems in a fitting model that are unclear when plotting the fit and looking at the log file become quite transparent when looking at the real/imaginary parts of each contributing path. To plot things, they need to be selected. In the language of Artemis "selected" means "highlighted in orange". You can select multiple entries in the Data and Paths List by holding down the control key while clicking the left mouse button on items in the list. Other mouse event combinations that are useful for doing extended selection are shift-click and click-drag. Paths also must be "included" in the fit to be plotted. The included ones are the ones written in black text. The excluded ones are the ones written in brown text. HTH, B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/