The two-pass rotation indexer could keep a doubled-axis supercell over its own
primitive sub-cell: when the supercell won candidate (ci) order and the primitive
cleared the indexed-fraction hysteresis by less than the 0.05 "prefer-earlier"
margin, the supercell survived. On pding4_003 full data this gave P222 on a
65.6x131.3x173 cell instead of P422 on 65.6x65.6x173 - the orthorhombic metric
forecloses the 4-fold before the symmetry search ever runs. The FFT peak-finding
is correct (the primitive cell is among the candidates, just out-ranked).
Add a sub-cell override to the selection loop: adopt a later candidate that is a
genuinely smaller cell (> ROT_SUBCELL_VOLUME_RATIO=1.5x smaller volume; a doubling
is 2x) indexing at least as many spots (within ROT_SUBCELL_FRAC_SLACK=0.02). That
is the signature of a spurious doubling - the primitive always indexes >= its
integer multiple, whereas a real superstructure's larger cell indexes MORE (kept
by the existing clearly-more branch) and twins share the cell volume (untouched).
Full 24-crystal rotation battery (pre-fix vs post-fix): pding4_003 P222->P422
(cell halves to 65.6x65.6x173, R_meas 6.6% CC1/2 99.9% ISa 11.9), pding4_001
holds P422, all 22 other crystals bit-identical in space group.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 4.8
4c050b576c