Two defects in the Bravais character walk, both of which silently cost symmetry.
An impossible character MATCHED. The walk computes acos(cond_F/sqrt(A*B)); when the character is
geometrically impossible the argument leaves [-1,1], acos returns NaN, and the acceptance test
fabs(NaN - actual) > tol is FALSE - so the character is taken. One corpus crystal matched a
monoclinic-C character whose implied cos(gamma) is 1.086.
The type-boundary retry covered beta only. All three angles carry the Niggli boundary, and negating
two basis vectors supplements the OTHER two, so each angle needs its own flip. A crystal whose
reduced gamma sits at 89.900 degrees needs the alpha/beta flip and never got it.
Measured on 84369 exact lattices spanning all 14 Bravais classes: nothing is lost in any class, and
orthorhombic-I recovery rises from 60.3% to 87.2%. Over 440000 random and perturbed cells the first
change only ever demotes a monoclinic-C match to triclinic and the second only ever promotes out of
triclinic - it is a strict superset of the beta-only retry. On the 94-dataset corpus, two crystals
gain their correct point group and none regresses.
Both regression tests fail without the change.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lc5JG6kJqZoCWaoZ43JGTW
leonarski_fandClaude Opus 5
10873eee7a