Hi Matthew, Bruce, OK, I think it's a bug. Well, maybe it's just a consequence of a "known but not well advertised" problem. The chi(k) data you have doesn't start at k=0. By itself, this should be OK, and the fit seems to be mostly fine. There seems to be a bug in Ifeffit that the output fit chi(k) has the same number of data points as the input chi(k) data. In this case is incorrect -- it should go to the last data point in k. From looking at the code, I'm pretty sure this has no effect on the fit and is only an error in the output array. But the shortened chi_fit(k) will have an impact on the way Bruce is calculating the residual. Even if that were fixed, the residual would still be wrong, as the residual in Artemis is calculated as set(data0_res.k = data0.k) set(data0_res.chi = data0.chi - data0_fit.chi) and then Fourier transforming to R-space. Since the data and fit aren't aligned in k-space, the residual is incorrect. It should be set(data0_res.chi = data0.chi - interp(data0_fit.k, data0_fit.chi, data0.k)) or some variation on that. Sorry for the trouble. It's definitely a result of narrow-mindedly expecting that all chi(k) data comes from Ifeffit/Athena and will always start at k=0. We should get this fixed (it's mostly there!) so that data from other sources can be used. Right now, the easiest work-around is to pre-pack the chi(k) data with zeros (starting from k=0.00) --Matt