Fixing a space group told the merge which absences to apply but never told it which
basis to apply them in. Where the indexed lattice was already conventional the group
was simply stamped on it, so a primitive tetragonal cell asked to merge in a
C-centred orthorhombic group had that centring rule evaluated in a frame the
reflections were not in, and half of them were declared systematically absent. The
reindex that would have fixed this existed but was reachable only from the triclinic
arm.
So ask the character table for the group's own class. The Bravais search grows an
optional class filter - one continue that skips characters of the wrong class, one
answer of "none fits" when the metric cannot carry it - and the reindexing that
followed the triclinic arm is lifted out and offered to a fixed group whose centring
is not the indexed lattice's, mapping a trigonal-P request onto the hexagonal-P
setting it is described in. With no class asked for, both new statements are dead
and the search is what it was.
That splits the failing cases in two, and conflating them was what made this wrong in
both directions. A lattice that HAS a setting carrying the group is reindexed into
it: the tetragonal case above recovers every observation it had been discarding, and
a centred monoclinic one that had been merging from a primitive cell without any
reindex - which nothing had noticed - goes from an error model that could barely be
fitted to a healthy one. A lattice that genuinely has no such setting - a triclinic
metric several degrees from monoclinic-C, or an F-centred cubic one asked for
hexagonal-P, whose hexagonal description is R-centred - has no basis to be put in,
and every statistic computed from it is meaningless. Those now stop, naming the
group, its centring and the cell that was actually indexed, and they stop only after
the reindex has been tried, so a mistyped but reachable group is repaired rather than
rejected. The second pass keeps its existing flag-and-decline instead.
Separately, the geometry pre-pass predicted in the primitive lattice only when no
group was fixed. With a centred group fixed it integrated half the events, moved the
error model, and shifted the post-refined distance by more than a tenth of a
millimetre - enough, in a loop this sensitive, to send the second pass down the other
branch. It now predicts primitive there whatever the group, which is what it already
did de novo and which its discarded intensities have no opinion about; the one
dataset this cost its indexing rate recovers completely, and lands on the same
answer it reaches with no group given.
Thirty-one of thirty-eight pinned runs are bit-identical and none is worse. De novo
nothing changes at all, by construction and on the whole rotation test set.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016NNnL26LAvruQ9eLUUWvrJ
leonarski_fandClaude Opus 5