Hi Bruce, All, These issues are almost certainly due to some bug in Ifeffit that gets triggered with large data sets. As Bruce mentioned, Ifeffit internally keeps track of arrays and groups using a built-in memory cache. By trying to write portable Fortran, this was more-or-less a requirement (at least, it was several years ago when Ifeffit was first being written). I have a few tests for moving arrays around that pass for me without any trouble, but there are definitely occasional problems with "large sets of data" -- I've seen these myself. Unfortunately, I don't have a good simple test case that always causes a problem. If you have a project file that is "acting weird" and data is clearly being corrupted, please send it to me. --Matt