Dear all, I have the following problem: In some fits using Artemis (Demeter 0.9.25) I will get negative values for parameters where it makes no sense at all. For example, when trying to fit two first shell paths, Pt-Pt and Pt-O, for a Pt nanoparticle, sigma^2 for the Pt-O path becomes negative. Obviously, sigma^2 has to be a positive number. My questions: 1. Why does the fit algorithm do this? 2. What can I do to avoid this? I know that there are most likely significant problems with my fit if this behaviour occurs. As far as I know it's possible to impose restrictions in Artemis and if anyone could point me towards a tutorial explaining how to implement restrictions that might be a first step in the right direction. Currently, the chapter "Constraints and restraints" in the Artemis documentation is "Coming soon......". With kind regards, Felix