rugnux fits sigma^2 = a*sigma0^2 + (b*<I>)^2, so its `b` is a fraction of the intensity. XDS fits
sigma^2 = a*(sigma0^2 + b*I^2) and prints ISa = 1/sqrt(a*b). The two `a` are the same number, but the
two `b` are not - b_xds = b^2/a - so the pair rugnux printed could not be read against a CORRECT.LP,
which is the only reason anyone looks at it.
Convert at the report. The fit, the merge weights and both engines' variance expressions are
untouched, so this is a re-expression and not a change: on a rotation dataset the merged intensities
move strictly less between before and after than they do between two runs of the SAME binary (99.9%
identical, max |dI/I| 9.1e-4 against the run-to-run control's 7.5e-3), with the same reflection set.
The rotation path also printed the wrong ISa for the comparison it invites. What it calls ISa is the
strong-reflection asymptote, a tier XDS has no equivalent of and which can only ever be the more
optimistic of the two; XDS's ISa is the whole-range 1/sqrt(a*b), which in rugnux units is exactly
1/b. Print both, labelled. On a broadband rotation dataset that is 13.2 (whole range) and 15.6
(asymptote) against XDS's 21.18 - so the number previously compared was flattering rugnux by 2.4.
A third, unrelated `b` lives in the space-group search: fitted with the sigma^2 coefficient held at 1,
with gate constants calibrated in that convention, and a ratio bound does not survive the mapping
(1.90 would have to become 3.61) while the absolute floor has no correct value at all, there being no
`a`. It is now commented as such, since making the three consistent is the obvious wrong move.
Also corrects three comments and two doc passages that still described a merged-sigma systematic
floor deleted in 72efb75a8.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 5
ac06b5c64f
