The monoclinic 2_1 screw was missed on EP_cs_02-10 and EP_cs_01-17 (adopted P2
instead of P2_1). The 0k0-odd reflections are correctly measured as weak (~0.1-1%
of 0k0-even, matching XDS), but their merged sigmas are ~2x too small, so their
I/sigma clears the present_i_over_sigma cut and they count as screw-axis
violations - the search then falls back to the symmorphic group.
Add a resolution-normalised intensity E^2 = I / <I>(shell), computed from
equal-count resolution shells, and require a reflection to reach present_e_squared
(0.3) as well as present_i_over_sigma before it counts as violating a predicted
absence. This only tightens "present", so it cannot manufacture a screw whose
predicted-absent class carries real intensity (a symmorphic crystal's axial
reflections sit at E^2 ~ 1 and still register as violations).
On the 18-crystal rotation battery this recovers the screw on EP_cs_02-10 and
EP_cs_01-17 (-> P2_1) and, as a side effect, on MyoB (-> P2_1), pding4_001
(-> P4_122/P4_322) and pding4_003 (-> P2_2_2_1) - all confirmed by genuine absences
in the reference intensities (absent class at 0.02-0.76% of the allowed class),
which the old sigma-only test also missed. The other 13 crystals, indexing rate
and ISa are unchanged; the screw-free control (Ins_I -> I23/I2_13) is unaffected.
Add a regression test that reproduces the under-estimated-sigma screw and checks
the gate recovers it (and that disabling the gate reproduces the miss).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
leonarski_fandClaude Opus 4.8
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