Where pass 1's post-refinement leaves the distance at the header but its free fit asks for a move of a
step (1 %) or more, the distance is now asked for as a fixed point: pre-passes at distances along the free
fit's move (the direct beam held where the free fit put it), the move each one asks for against the
distance it was integrated at, and the zero of the least-squares line through them - taken only where the
distances tried bracket it (interpolated, never extrapolated). That distance is integrated once more and
adopted only where it realises a lower held-out residual than the header pass, beyond its noise, or the
validation frames prefer it - the tests a round of the geometry walk passes. Otherwise the header stands.
Why: a fit is measured on reflections integrated at the geometry it starts from and is pulled towards it.
On a detector swung out on a 2theta arm with a header 4 % short, the fit asked for 104.4 mm from data
integrated at 103 and 108.6 mm from data integrated at 109; spot-finder centroids in place of the
integrated ones moved neither. Each round of the existing walk closes a third of the gap and the walk
stopped 3 mm short. Measured (CPU, from the headers, no -C):
- two 2theta = 35 deg sweeps of one cubic crystal: 103 -> 108.0 / 107.5 mm, cell 74.4 -> 77.8 A,
ISa 7.2 -> 13.4 and 4.5 -> 25.9, the second's space group back from P212121 to the cubic group;
- a 2theta = 45 deg sweep of an orthorhombic crystal: 183 -> 189.9 mm, cell 4.5 % -> 0.4 % short,
ISa 4.2 -> 8.2, R_meas 32 -> 18 %;
- low-resolution sweeps at 2theta = 10 deg ask for +5.5 mm from every distance between 183 and 205 mm
and bracket nothing: header kept, output bit-identical;
- eight controls whose header is right (free fit within 0.3 %): output bit-identical.
The beam the post-refinement fits is the PONI; on a swung detector it moves with the distance at a fixed
direct beam (7 px per mm at 35 deg), so the 15 px bound on it capped the distance near 105 mm. The bound,
and the comparison with a measured centre (which measures the direct beam), now apply to the direct beam;
on an untilted detector nothing changes.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HVeDQqrcrEZy6XBUCMZvyT
leonarski_fandClaude Opus 5.5
2dd45a1ca2