Hi all, I have modified the least square residual classes in cctbx/xray/ targets.h and cctbx.xray to handle refinement against F^2. In the subsequent tests, the following assertion failure cropped up. It does not feel to me my changes caused it… Luc libtbx.python /Users/luc/Developer/cctbx/cctbx_sources/cctbx/cctbx/ regression/tst_twin_target.py approx_equal eps: 0.5 approx_equal multiplier: 10000000000.0 -0.510137170481 approx_equal ERROR 0 approx_equal ERROR Traceback (most recent call last): File "/Users/luc/Developer/cctbx/cctbx_sources/cctbx/cctbx/ regression/tst_twin_target.py", line 540, in <module> run() File "/Users/luc/Developer/cctbx/cctbx_sources/cctbx/cctbx/ regression/tst_twin_target.py", line 488, in run tst_ls_on_i() File "/Users/luc/Developer/cctbx/cctbx_sources/cctbx/cctbx/ regression/tst_twin_target.py", line 95, in tst_ls_on_i assert approx_equal( (fdif_real-old_derivs[0][N_test])/fdif_real, 0, eps=5e-1) AssertionError