The systematic-b test (a genuine symmetry step barely moves the merge error
model's b; a merohedral twin, forcing non-equivalent reflections together,
balloons it) was only ever a RESCUE - it could promote a chi^2-borderline
genuine step but never demote a chi^2-passing one. So a partial twin whose
within-orbit scatter looks self-consistent (merged chi^2 below the ratio
bound) slipped through on chi^2 alone and was over-promoted to the holohedral
group, ballooning b and collapsing the merged ISa.
Make the same test a VETO: a chi^2-passing high-symmetry promotion whose b,
relative to the largest confirmed subgroup, exceeds max_systematic_b_veto is
kept in its true lower symmetry. Calibrated on the rotation-test battery,
where the largest genuine step (a P422 tetragonal) sits at b-ratio ~1.8 and a
merohedral R3->R32 twin at ~2.6 - an empty gap - so 2.0 catches the twin while
never demoting a genuine high-symmetry merge.
Battery space-group match 20->21/25 (an R3 case previously merged as R32 is
now correct), every other crystal unchanged.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 4.8
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