De-novo space-group search:
- Determine lattice centering from the strength of the systematically-absent
class (its mean I/sigma vs the present class) instead of a per-reflection
violation count. The count gate was brittle on noisy / obverse-reverse-
twinned data, where enough genuinely-absent reflections cross I/sigma>3 to
trip the 10% bound although the class is several-fold weaker, and wrongly
kept a genuine R-centered lattice primitive. False centerings, whose absent
class is as strong as the present one, stay rejected. Screws keep the count
test (too few axial reflections to average).
- Recover a genuine high-symmetry point group when the merged error model is
badly miscalibrated (weak, low-resolution data whose sigmas are far too
small). The fixed-sigma chi^2 ratio then grows with point-group order for
real symmetry too and wrongly rejects it (a true cubic group reached a
chi^2 ratio ~14); the self-normalising systematic-b test stays valid, so
once chi2_ref shows the ratio is untrustworthy the promotion is confirmed
on the b-test alone. The balloon veto still rejects a twin. This is the gap
left when the earlier log10(chi2_ref) widening was removed for the b-veto.
Also print the adopted space group and unit cell as a clean one-line summary
at the end of the run (de-novo or user-fixed -S), not only in the mmCIF.
Full rotation-test battery: exactly the two intended crystals change (both to
the correct answer, with lower R-meas and higher ISa), every other crystal
unchanged; space-group match 21->22/25.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 4.8
67a18357b5
