While testing the new simTBX diffraction image simulator Aaron and I found that some rather beautiful-looking images still won't index. Subsequently, I followed up with mosflm, which does succeed in these cases, even without prior knowledge of the cell or space group. I have now run this 1000 times, varying only the crystal orientation, and found 171 cases of dials.stills_process failing to index the simulated image whereas mosflm had no trouble, and 5 cases where mosflm failed and dials.stills_process succeeded. There were no overlapping cases where both programs failed. I have tarballed up the relevant files here: http://bl831.als.lbl.gov/~jamesh/bugreports/dials_index_020218.tgz The tarball contains a "runme.com" shell script for reproducing these results, and also the tst_nanoBragg_forindex.py jiffy for making a test image given a provided random-number seed. For example: libtbx.python ./tst_nanoBragg_forindex.py random 15 will create a file called noiseimage_001.cbf that dials.stills_process cannot index (at least in my hands). For those of you who don't have mosflm installed, the tarball incluses a copy, and the "autoindex.com" script should run on any linux system. Any ideas as to why there is this discrepancy? Feels like an opportunity for a non-trivial improvement. Something to ponder over the weekend, I suppose. Note that I recently checked in a few bug fixes to simTBX, but this analysis is unaffected by them, so you shouldn't have to update your build to reproduce this. Cheers, -James Holton MAD Scientist