Hi Nic, The delr and ss values may adopt reasonable values with a negative amp, but they're almost certainly not the right values. A negative amp turns the chi(k) upside-down. To make it ft, the other parameters then have to shift the graph over by half an oscillation, yielding values of E0 and delR that are wrong. It is unusual that a first-shell fit to a standard doesn't seem to be giving a qualitatively reasonable result with an amp of 1. What happens if you do a sum of the most important paths without a fit? Does the fit look qualitatively correct over a large range of R? (The amplitudes will of course be substantially off without a fit.) It sounds like something substantial is wrong: either the material isn't actually dadolinium oxide, or the structure you're using for the feff calculation isn't right (is there more than one crystallographic setting?), or there was serious distortion in the measurement (perhaps a thick, concentrated standard was measured in fluorescence without correction). --Scott Calvin Sarah Lawrence College On Aug 1, 2011, at 12:36 AM, Nicholas.Tse@csiro.au wrote:

Dear All,

Just wondering what could I do to make the amp guess value as a positive number? If I run it and let it float (i.e. guess), the amp becomes negative in the resulting fits, but the fit has nice delr and ss values (i.e. make sense). If I restrict the amp value to 1, everything else doesn’t fit. I am only fitting the first nearest neighbour in a measured standard; Gadolinium oxide.

Regards,

Nic