Hi Abhijeet, The guess values are simply the starting point for ifeffit to work from, so they don't matter as long as they're "reasonable," which the default values are, at least if your normalization and background subtraction is OK. But more importantly, the default parameters themselves are not appropriate for more than one shell of copper foil! If you're using more than one path, you need all the paths to expand by the same fraction, not the same number of angstroms. Therefore you want to guess a parameter (let's call it "alpha") and make the delr for each path alpha*reff. (ifeffit knows that reff means half the nominal length of that path.) Likewise, it's probably inappropriate to use the same sigma2 for all paths, but fortunately for metals the Debye model works fairly well. You can have a guessed variable for the Debye temperature (say, "omega"), and then have sigma2 for each path be Debye(temp, theta), where "temp" is the temperature you measured the data at. As far as the ideal fitting range in R for copper foil, you can usually manage out to 5 angstroms or so with a metal foil without too much difficulty. --Scott Calvin Sarah Lawrence College On Jan 29, 2009, at 5:11 AM, abhijeet gaur wrote:

Hi I am fitting a copper foil whose data is taken at room temperature. So do I need to change the guess value of parameters enot, ss,amp, delr to some other value or I should take them as the default guess value. Also what should be the ideal fitting range in R for the copper foil.

With thanks Abhijeet