Whether a twin law can exist was read off a hard-coded list of Laue classes
(4/mmm, 6/mmm, m-3m and rhombohedral -3m "holohedral", everything else
eligible). That is a property of the lattice, not of the class: a C-centred
orthorhombic or a monoclinic lattice with no special metric admits no twin law
for mmm or 2/m either, and a pseudo-merohedral one admits a law the class says
nothing about. On five pseudo-merohedral sets the report said "Twinning
INDICATED - refinement needs a twin law" where the lattice admits none.
TwinLawPossible asks gemmi's find_twin_laws (Le Page two-folds within 3 deg,
as RfreeFlags and ReindexAmbiguity already do), and AnalyzeTwinning takes the
cell to decide it; in P1 or without a cell a law stays possible, so the
pre-promotion analysis and the space-group search are unchanged. Twin-like
statistics (<|L|> below 0.44) above the false-operator bar where no law is
possible are now reported as NO_TWIN_LAW: not a twin in this group, so the
symmetry may be too high or something else narrowed the distribution.
Report-only: no reflection file changes.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SVmAWnzCmRKAXVUCdc4iNi
leonarski_fandClaude Opus 5.5
8f05793b67