I am a relative novice using TOPAS (Version 6), and this is my first post, so please be gentle. I have come across a specific problem chaining two xdds in an INP file. I am fitting two poorly crystalline anisotropic phases with overlapping peaks (carbonated apatite and whitlockite), and I have found the following strategy to work very well interactively. The first stage involves an hkl phase and a Str phase across the whole pattern to calculate the lattice parameters. The second stage involves a Peaks fit on a limited window with one of the phases, with fixed positions calculated from the lattice parameters, to extract the individual peak parameters. This works well both interactively and automated with an INP program in cases where there is very little whitlockite (i.e. just feeding the apatite peak positions to the second stage), but I need to include the whitlockite phase in the second stage in cases where this has a substantial presence, with fixed parameters as calculated in the first stage. I can do that interactively, but if I put this into the second xdd I get the following error: "Cannot locate r_bragg from lor_fwhm in data structures". I need to automate it because I have thousands of diffractograms to analyse. Has anyone come across this error message before and could give a steer as to how I might fix it?