Jungfraujoch/docs/RUGNUX_INTEGRATION.md

# Rugnux with other programs

What the reflection files promise to a reading program, and the minimum commands that get each
downstream suite running on Rugnux output.

```{contents} On this page
:local:
:depth: 2
```

## Reflection-file conventions

**mmCIF.** Standard items carry their standard meanings — `_refln.intensity_meas` / `_intensity_sigma`,
the `pdbx_I_plus`/`pdbx_I_minus` and `pdbx_F_plus`/`pdbx_F_minus` anomalous pairs, `_reflns.*` and
`_reflns_shell.*` for the merging statistics, `_reflns.B_iso_Wilson_estimate` for the Wilson B, and
`_cell.*` / `_diffrn_radiation_wavelength.wavelength` for the geometry.

Anything Rugnux reports that has no standard item is written under a **`jfjoch_`** prefix, inside the
standard category it belongs to. That is a deliberate choice: a reader that does not know these items
ignores them, and one that does can find them without guessing.

| item | meaning |
|---|---|
| `_reflns.jfjoch_diffrn_ISa` | Asymptotic I/σ in **XDS's sense**: the whole-range `1/√(a·b)` of the error model, so it can be read directly against a `CORRECT.LP` |
| `_reflns.jfjoch_diffrn_ISa_asymptotic` | The **strong-reflection** tier — the counting-subtracted scatter of well-measured groups. XDS has no equivalent, and it can only ever be the more optimistic of the two. Rotation path only |
| `_reflns.jfjoch_error_model_a`, `_b` | The error model in XDS's convention, `σ² = a(σ₀² + b·I²)`, so the ISa above is re-derivable from the file rather than taken on trust |
| `_reflns.jfjoch_second_moment_I` | Twinning second moment ⟨I²⟩/⟨I⟩² — 2.00 untwinned, 1.50 for a perfect twin |
| `_reflns.jfjoch_L_test_mean_abs_L`, `_L_test_mean_L_squared` | Padilla–Yeates L-test. ⟨\|L\|⟩ is 0.500 untwinned / 0.375 for a perfect twin; ⟨L²⟩ is 0.333 / 0.200. Written only when the test found pairs |
| `_reflns.jfjoch_radiation_damage_relative_B` | Relative *B* from the first to the last rotation batch (Ų); positive is the usual direction, high-resolution intensity fading with dose |
| `_jfjoch_radiation_damage_batch.*` | Per-batch loop: `id`, `rotation_start_deg`, `relative_B` |
| `_diffrn_detector.jfjoch_distance_mm`, `_jfjoch_beam_center_x_pxl`, `_jfjoch_beam_center_y_pxl` | The refined detector geometry actually used, which is not otherwise recoverable from the reflection file |
| `_reflns.pdbx_aniso_B_tensor_eigenvalue_1..3`, `_pdbx_aniso_B_tensor_eigenvector_*` | The anisotropy tensor, eigen-decomposed. Eigenvalues are **relative to the weakest direction** (so the third is 0 and the first is the anisotropic Δ*B*), because only the deviatoric part is determined; eigenvectors are in the PDB orthogonalisation convention. Not written for a cubic Laue class, where symmetry forces Δ*B* to be zero |
| `_reflns.jfjoch_aniso_delta_B`, `_jfjoch_aniso_delta_B_linear` | The anisotropic Δ*B*, and the part of it that actually follows exp(−½ **s**ᵀ*B***s**). The second is what the verdict is gated on |
| `_reflns.jfjoch_aniso_d_min_1..3` | Diffraction limit (Å) along each principal direction. A comment marks a value that is the edge of the measured data rather than the crystal's own limit |
| `_reflns.jfjoch_aniso_shape`, `_jfjoch_aniso_floor`, `_jfjoch_aniso_significance`, `_jfjoch_aniso_verdict` | The resolution signature of the deficit, the data set's own systematic-error floor, Δ*B*<sub>linear</sub> over that floor, and the resulting verdict. Each carries its vocabulary as a comment |

> **Compatibility note.** Before rc.161, `_reflns.jfjoch_diffrn_ISa` carried the *asymptote*, not the
> whole-range value. There is no version marker inside the file, so a number taken from an older
> `.cif` is not comparable with one taken from a newer one.

**SHELX HKLF 4** (`<prefix>.hkl`). Fixed-format `3I4,2F8.2` — `h k l I σ(I)`, one record per
reflection, terminated by a `0 0 0` record — which is what **SHELXC**, **SHELXD** and **ANODE**
expect. Two properties worth knowing before using it:

- **Bijvoet mates are written separately**, `I(+)` at `+hkl` and `I(-)` at `-hkl`, so the anomalous
  differences survive into SHELXC; a reflection with no anomalous split is written once, as its mean.
- **Intensities are rescaled** by a single global factor so the largest value fits the `F8.2` field.
  `I` and `σ(I)` share that factor, so every ratio — and therefore the anomalous signal — is
  untouched, but the absolute scale is not meaningful. This matters only if you intend to compare
  magnitudes with another file; SHELXC and ANODE use ratios alone.

**MTZ** (`<prefix>.mtz`, and `<prefix>_P1.mtz` beside it). The CCP4 anomalous layout, with the
column types CCP4 programs dispatch on:

```
H K L IMEAN SIGIMEAN I(+) SIGI(+) I(-) SIGI(-) F SIGF F(+) SIGF(+) F(-) SIGF(-) FreeR_flag
H H H    J        Q    K       M    K       M  F    Q    G       L    G       L          I
```

`F` is the French–Wilson amplitude. The header carries the determined space group, the refined cell
and the wavelength, on a dataset of its own behind the reserved `HKL_base`, which is where the MTZ
format puts them. Older Rugnux wrote the data on dataset 0, the id reserved for `HKL_base`, and
CCP4's `mtzinfo` then reported its 1.54187 Å (Cu Kα) default instead of the real wavelength — every
other reader tried, `mtzdmp`, `truncate`, `ctruncate`, gemmi, iotbx and `phenix.xtriage`, recovered
the true value from those files as well, so the effect was confined to that one report. Note that
`<prefix>_unmerged.mtz` still reads 1.54187 under `mtzinfo` and is not wrong: its columns sit on
`HKL_base` deliberately, as POINTLESS expects, and the wavelength AIMLESS and POINTLESS read is the
per-batch one, which is correct. The Bijvoet columns are present on any rotation merge, with or
without `-A`; a stills merge has no Bijvoet split and the file then stops after `F SIGF FreeR_flag`.

There is deliberately **no `DANO`/`SIGDANO`** pair, the anomalous difference columns a CCP4 merged
file usually carries. They are a restatement rather than a measurement: checked column against
column on a `ctruncate` file, `DANO` is `F(+) − F(-)` to the last bit and `SIGDANO` is
`√(σ(+)² + σ(−)²)` to the last bit, on every reflection — the quadrature sum is the convention
whether or not the two mates came from one scale model, and no correlation correction is applied by
anybody. Every program in the phasing routes below reads the Bijvoet columns directly and forms the
difference itself, and CCP4's own phasing engines prefer them: `bp3` and `afro` want `F+/SF+/F-/SF-`
and tell a user holding `F`/`DANO` to convert *to* that form, and `mtz2sca` ranks `I(+/-)` over
`F(+/-)` over `F/DANO`. Where the pair is genuinely wanted — `fft`'s anomalous-difference Fourier
takes a `DANO` label and has no other spelling — one command makes it, with the `ISYM` column that
belongs beside it:

```
ctruncate -hklin myrun.mtz -hklout myrun_ct.mtz \
    -colin '/*/*/[IMEAN,SIGIMEAN]' -colano '/*/*/[I(+),SIGI(+),I(-),SIGI(-)]'
```

## The unmerged export

`<prefix>_unmerged.mtz` holds every integrated observation, before scaling and merging, in the column
layout POINTLESS writes and **aimless**, **pointless**, **careless** and `iotbx.merging_statistics`
read. It is written by default, in `--mode mx` and `--mode scale` alike and with `--no-merge` as
well, and it replaces nothing — Rugnux still writes its own merged files in the same run. It needs an
output prefix (`-o`). It is the largest file a run produces, larger on a dense rotation dataset than
the merged `.mtz`, `.cif` and `.hkl` put together, so a run that only wants the merged numbers — a
regression battery, or a throughput pipeline — turns it off with `--no-export-unmerged`.

Use it to scale the data with a different program, to have pointless give an independent opinion on
the space group, or to compare Rugnux's merge against another one on identical input. Each sweep's
file is self-contained, so several of them can be handed to pointless and aimless as separate
`HKLIN`s to merge sweeps Rugnux does not combine itself.

> **Trap when combining a wild-carded series.** For an `HKLIN` given with wild-cards, POINTLESS
> accepts the files in order and **terminates acceptance at the first file out of chronological
> order**, then merges what it kept and prints a plausible result. Its own keyword lifts the check —
> `ALLOW OUTOFSEQUENCEFILES` — or name each file as its own `HKLIN`, which is not a series; either
> way, check the file count in its log against the number you meant to give.

**Columns.** `H K L M/ISYM BATCH I SIGI FRACTIONCALC XDET YDET ROT LP QE FLIGHT FLAG` —
POINTLESS's own set down to `FLAG`, plus `QE` and `FLIGHT` (the sensor-efficiency and flight-path
divisors described above; `QE` is DIALS's column) — then four Rugnux extras, `DELPHI` (offset from
the centre of the rocking curve), `ZETA` (the Lorentz geometry of that curve), `BGMEAN` and `BGVAR`
(the background that was subtracted, and its variance).
`BATCH` is the image ordinal plus one, and a batch header is written for every batch that carries an
observation. `M/ISYM` records both the symmetry operation and the Friedel hand, so the index as
measured is recoverable from the index as stored.

**Header symmetry and order.** The file's MTZ header carries the **space group the run determined**
(P1 where none was), and the rows are **sorted on `H K L M/ISYM BATCH`** — the order POINTLESS
leaves an unmerged file in, and the order AIMLESS requires of its input — so both programs take the
file directly.

**What has been applied to the intensities, and what has not.** `I` and `SIGI` carry the
**deterministic per-reflection corrections and nothing else**. Three columns record them: `LP` is
Lorentz x polarization; `QE` is the sensor's quantum efficiency at the angle the diffracted beam
meets the detector; and `FLIGHT` is the attenuation in the medium the reflection crossed on its way
there. `QE` and `FLIGHT` are both **divisors** normalised to 1 at normal incidence, so raw counts are
`I / LP * QE * FLIGHT`. They are applied because they are per-observation geometry that varies by
more than two orders of magnitude across a sweep and no reader can reconstruct them. `QE` is at least
1 and `FLIGHT` at most 1: an oblique reflection crosses more sensor, which makes it read high, and
more of the medium, which makes it read low. `FLIGHT` is a column of ones under
`--flight-path vacuum`.

`QE` is kept out of `LP` because that is what the field means by `LP`: XDS's `RLP` is Lorentz x
polarization alone (its own column is flat to 0.1% across a detector over which the efficiency term
spans 7%), and DIALS fills `LP` from lorentz/polarization only and writes `QE` as a separate column —
a column of ones where it has no correction. Rugnux normalises `QE` to normal incidence where DIALS
stores the un-normalised absorbed fraction; the two differ by a per-dataset constant, i.e. by an
overall scale. Deliberately *not* applied:
the **partiality is not divided out** (it is reported in `FRACTIONCALC`), and the **per-image scale is
not applied at all** — those programs fit their own scale model, and handing them pre-scaled data
would have them fit a correction to a correction. No resolution cut, outlier rejection or ice-ring
filtering is applied either.

**Partials.** On a rotation run the partials of each reflection are summed into one full, using the
same rule Rugnux's own 3D combine uses — consecutive frames no more than two apart — and the full is
written at the batch its rocking curve is centred on, with the summed rocking-curve fraction in
`FRACTIONCALC`. An event that caught less of its rocking curve than `--min-captured-fraction` (or `--min-partiality`) is not
written, exactly as in the merge. Summing is the default because a downstream program's own partial
handling is far more conservative than Rugnux's: given raw partials, aimless accepted a small
fraction of the file and merged at a fraction of the multiplicity; given summed fulls it uses
essentially all of it. `--export-unmerged-partials` writes the unsummed form to
`<prefix>_unmerged_partials.mtz` for a program that would rather sum them itself. Stills have no
rocking events and are the same either way.

**Systematic absences.** Lattice-**centring** absences are not written; **screw and glide** absences
are. Prediction runs in a primitive setting so that the space-group search can test the centring,
but the interstitial reflections that leaves make a reading program take the lattice for primitive
and demote the group. Screw and glide absences are kept because they are the evidence the space
group was chosen on — deleting them would turn a reading program's test into an assumption. XDS and
DIALS draw the line in the same place.

**Scan axis.** The batch headers carry the goniometer axis **negated** relative to the one in the
input file. This is not a correction to the file: Rugnux brings an observation made at angle φ back
to zero by rotating it by +φ, so the crystal itself turns by −φ, and an MTZ batch header records the
axis a batch's own increasing `PHI` turns the crystal about. With the sign as exported, pointless's
independently determined orientation matrix agrees with Rugnux's to well under a degree.

## Taking the data onward

The reflection files are inputs to other suites, and the handover has a few conventions worth one
line each. These are the minimum commands that get each program running on Rugnux output.

**phenix.** The merged files carry both the mean intensity and the Bijvoet pairs, and a phenix
program that has not said which it wants stops on the pair of them — from the MTZ and from the
mmCIF alike, each listing its own format's labels:

```
Sorry: Multiple equally suitable arrays of observed xray data found.

Possible choices:
  myrun.mtz:IMEAN,SIGIMEAN
  myrun.mtz:I(+),SIGI(+),I(-),SIGI(-)
```

Two things are worth knowing before reading that as a fault in the file. The tie is between the two
**intensity** arrays and nothing else: iotbx scores `F`/`SIGF` and `F(+)`/`F(-)` below them, so they
are never in the running and writing amplitudes as well as intensities is not what causes this. And
**ctruncate's own output ties in the same place** — put any merged data through CCP4's truncate step
and phenix asks the same question of the result, because a mean intensity array and an anomalous one
score equally whenever the calling program has expressed no preference. The only file change that
removes the tie is dropping one of the two, and dropping the Bijvoet columns would take the anomalous
signal — and the whole [SHELX route](#experimental-phasing-with-shelx) — with it.

So the answer is a label. The parameter name differs by program, which is the part that catches
people out:

```
phenix.xtriage myrun.mtz xray_data.obs_labels=IMEAN
phenix.xtriage myrun.mtz "xray_data.obs_labels=I(+)"        # the Bijvoet array instead
phenix.refine  model.pdb myrun.mtz miller_array.labels.name=IMEAN
```

`IMEAN` on its own is enough — the match is on a substring — and `IMEAN,SIGIMEAN` and the
fully-qualified `scaling.input.xray_data.obs_labels=` work equally. Quote the anomalous one: the
parentheses are shell syntax otherwise. The same behaviour appears on the mmCIF in that format's own
vocabulary, and a label from one format does not work on the other (`Sorry: No matching array`):

```
phenix.xtriage myrun.cif xray_data.obs_labels=intensity_meas
phenix.xtriage myrun.cif xray_data.obs_labels=pdbx_I_plus
```

**A program that states a preference needs none of this.** `phenix.hyss`, `phenix.find_peaks_holes`,
`phenix.molprobity` and the data import behind `phenix.autosol` ask for anomalous data by preference,
which breaks the tie for them. `phenix.hyss myrun.mtz n_sites=6 scattering_type=S` opens the file
with no labels given, reports `Miller array info: myrun.mtz:I(+),SIGI(+),I(-),SIGI(-)`, and forms the
anomalous differences itself.

**The R-free convention.** `FreeR_flag` is **0 = free, 1 = work** — the CCP4 convention the
column's own name belongs to (5 % free by default). REFMAC5's default `FREE 0` reads it directly
and phenix.refine detects the numbering on its own, so neither needs a keyword:

```
refmac5 XYZIN model.pdb HKLIN myrun.mtz XYZOUT refined.pdb HKLOUT refined.mtz <<eof
LABIN FP=F SIGFP=SIGF FREE=FreeR_flag
NCYC 10
END
eof
```

(A merged MTZ written before rc.166 carried the opposite, phenix/CNS numbering — 0 = work — under
the same column name; REFMAC5 stops on such a file with `more than half of reflections are in free
R set` and `Cannot switch free R flag`, and the keyword `FREE 1` is the cure for those files only.)

**POINTLESS / AIMLESS.** `myrun_unmerged.mtz` opens in both directly — it is sorted the way AIMLESS
requires and its header carries the determined space group (see
[The unmerged export](#the-unmerged-export)). Running pointless first remains the safe route, and
its independent space-group opinion is what the file exists for:

```
pointless HKLIN myrun_unmerged.mtz HKLOUT sorted.mtz
aimless   HKLIN sorted.mtz         HKLOUT scaled.mtz
```

Several sweeps of one crystal form go in as separate `HKLIN`s to the same pointless run — that is
how sweeps Rugnux does not combine itself are merged.

**careless** wants exactly what the unmerged export is — unmerged, unscaled intensities carrying
only the deterministic per-reflection corrections, with the partiality reported and not divided
out. Against its published examples, two renames: `BG`/`SIGBG` are called `BGMEAN`/`BGVAR` here
**and `BGVAR` is a variance, not a sigma**. A `QE` column is present, as in DIALS output
(normalised to 1 at normal incidence where DIALS stores the absorbed fraction — a per-dataset
overall scale). `Hobs`/`Kobs`/`Lobs` are reconstructed from `M/ISYM` by reciprocalspaceship, and
`dHKL` careless computes from the cell, so the metadata string that names this file's columns is

```
careless mono --anomalous "BATCH,dHKL,Hobs,Kobs,Lobs,XDET,YDET,BGMEAN,BGVAR,LP,FRACTIONCALC" \
    myrun_unmerged.mtz out/myrun
```

**Molecular replacement and experimental phasing** each get a section of their own below —
[Phaser](#molecular-replacement-with-phaser) and [SHELXC/D/E](#experimental-phasing-with-shelx).
Both are where Rugnux stops and the next program starts, and both meet the one thing the merged
intensities could not decide: which of several space groups the data are in.

**`iotbx.merging_statistics myrun_unmerged.mtz`** needs no arguments or label choices at all.

## Molecular replacement with Phaser

rugnux does not do molecular replacement, so Phaser is the next program for anyone who has a search
model. Both CCP4 and phenix ship it — `phaser` and `phenix.phaser`, the same 2.8.3 build in the
versions this was checked against — and either takes the merged `myrun.mtz` as it is written.

**No `LABIN`, no label choices.** Phaser reads the cell, the space group and the resolution range
out of the file and picks the intensity columns itself. Where phenix stops on a merged file because
it cannot choose between two equally usable observation arrays (see above), Phaser simply announces
what it took:

```
   Data read from mtz file: myrun.mtz
   Space-Group Name (Hall Symbol): P 41 21 2 ( P 4abw 2nw)
   Unit Cell:   78.06   78.06   37.70   90.00   90.00   90.00
   Column Labels Selected: IMEAN SIGIMEAN
   Resolution on Mtz file:  0.99 39.03
```

So the whole run is the model and the cell contents:

```
phaser <<eof
MODE MR_AUTO
HKLIN myrun.mtz
ENSEMBLE model PDBFILE model.pdb IDENTITY 1.0
COMPOSITION PROTEIN MW 14300 NUMBER 1
SEARCH ENSEMBLE model NUMBER 1
ROOT myrun_mr
eof
```

On a 1.0 Å dataset in a tetragonal point group that run placed one copy at TFZ 11.1, refining to
TFZ== 80.3 and LLG 10247, in 54 s of wall clock, with no warnings about the file. The one trap in
that script has nothing to do with Rugnux: `COMPOSITION PROTEIN SEQUENCE` wants a **file name**, and
given a chain identifier instead it fails with `FILE OPENING ERROR: X` before it reads anything. Use
`MW` unless you have the sequence file to hand.

**The space group is the interesting part.** `SPACE_GROUP_NAME` in the results report is a scalar
and reads like a determination, but it is one of the groups the absences allow, chosen by
convention — section 2 says which others it could not separate, as `SPACE_GROUP_ALTERNATIVES`,
and whether the hand is open, as `SPACE_GROUP_ENANTIOMORPH= UNDETERMINED`. Merged intensities never
name a hand: an enantiomorphic pair has the same absences and the same Laue class. Phaser is one of
the few programs that can settle it, because a wrong hand simply fails to place the model.

It does this **without being asked**. `MODE MR_AUTO` defaults to `SGALTERNATIVE SELECT HAND`, so the
run above listed

```
   Space Group(s) to be tested:
     P 43 21 2
     P 41 21 2
```

and returned a single solution in `P 43 21 2` — the hand opposite the one in the MTZ header.
Nothing in the command asked for that. The space group of the solution is the answer, whichever hand
the file happened to carry, and it is on the `SOLU SPAC` line of the `.sol` file and in the `CRYST1`
of the placed model.

**When the alternative is not the hand**, name it. `SPACE_GROUP_ALTERNATIVES` also carries screw
variants that share a point group — `I 2 3` and `I 21 3` on a body-centred cubic lattice is the
common one — and `SGALTERNATIVE SELECT ALL` searches every group Phaser derives from the input one
by translation symmetry. On a `P 41 21 2` input that is all eight of `P 4 2 2` … `P 43 21 2`, and it
took the run above from 54 s to 65 s; on an `I 2 3` input it is `I 2 3`, `I 21 3` and an
origin-shifted `I 2 3`. To see the list a given file would produce without searching it,
`MODE CCA` prints it and stops:

```
phaser <<eof
MODE CCA
HKLIN myrun.mtz
COMPOSITION PROTEIN MW 14300 NUMBER 1
ROOT myrun_cca
eof
```

**What Phaser cannot repair from this file** is a wrong *point* group. `SGALTERNATIVE` moves within
one, so a run whose report carries a non-`NONE` `SPACE_GROUP_REFUSED_POINT_GROUP`, or a point group
you suspect is too high, has to be merged again rather than searched again — `myrun_P1.mtz` is
written for exactly that, and `myrun_unmerged.mtz` will do it through pointless.

**mmCIF is not a route into Phaser.** `HKLIN myrun.cif` stops at `FILE OPENING ERROR: myrun.cif`,
in both the CCP4 and the phenix build — 2.8.3 reads MTZ only. Convert rather than look for a
keyword:

```
gemmi cif2mtz myrun.cif fromcif.mtz
```

That file gives the same solution — same space group, same placement to a hundredth of a degree,
LLG 10248 against 10247. Its amplitude columns come out as `FP`/`SIGFP` where Rugnux's own MTZ
writes `F`/`SIGF`, which matters only if you were naming columns by hand; the automatic choice is `IMEAN`/`SIGIMEAN` either way. Since Rugnux writes
the MTZ and the mmCIF in the same run, the conversion is only worth knowing about for a file that
arrived without its `.mtz`.

## Experimental phasing with SHELX

`shelxc`, `shelxd` and `shelxe` come with CCP4 (phenix does not ship them). The input is
**`myrun.hkl`**, and it is the only one of the three reflection files that works: SHELXC 2016/1
reads XDS and SHELX formats, not MTZ, and `SAD myrun.mtz` gets `** Cannot open file myrun.mtz **`
— after which SHELXC exits **0** and writes nothing, so a script has to check for the `_fa.hkl` it
should have produced rather than trust the exit status.

**Nothing has to be switched on to get the anomalous signal.** A default rotation merge keeps the
Bijvoet split, whether or not `-A` was given: `myrun.mtz` carries `I(+)`/`I(-)` and `F(+)`/`F(-)`
beside the means, and `myrun.hkl` writes each mate as its own record, `I(+)` at `+hkl` and `I(-)` at
`-hkl`. `-A` changes what the merging statistics are counted over, not whether the signal is in the
file. The one case with no anomalous columns at all is a **stills** run, which computes no Bijvoet
split; there `myrun.hkl` holds means only and there is nothing for SHELXC to work with. Unmerged
data are not wanted anywhere in this chain either, so a run with `--no-export-unmerged` is not
missing a file SHELX needs.

**HKLF 4 carries no metadata**, so the cell and the space group have to be repeated on the SHELXC
command — take them from `UNIT_CELL_CONSTANTS` and `SPACE_GROUP_NAME` in section 2 of the
report, with the spaces taken out of the group's name. (SHELXC also puts a wavelength in the `CELL`
line of the `.ins` files it writes; that is its own 0.98 Å default, not anything read from the data,
and neither SHELXD nor SHELXE uses it.) The whole chain, for a sulfur substructure — the cell and
group here are tetragonal lysozyme's, so substitute your own report's:

```
shelxc sad <<eof
SAD myrun.hkl
CELL 79.0 79.0 38.0 90 90 90
SPAG P41212
FIND 10
SFAC S
MAXM 2
eof
shelxd sad_fa
```

SHELXC's own table is the first honest look at whether this is worth continuing — `<d"/σ>` should be
about 0.80 where there is no anomalous signal. Two sweeps are quoted below, both collected at 5 keV
for the sulfur signal: a cubic one that went all the way, and a tetragonal one that did not. The
cubic one, 2.5 Å at 95 % completeness and multiplicity 30, reads:

```
 Resl.   Inf. 13.02  8.01  6.03  4.93  4.22  3.71  3.33  3.04  2.80  2.60  2.43
 <I/sig>   108.8  91.4  63.0  64.7  70.6  61.8  45.5  34.7  23.7  12.4   5.0
 %Complete  96.2 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0  99.0  72.7
 <d"/sig>   2.58  5.06  3.97  2.92  2.36  1.68  1.50  1.33  1.48  1.38  1.79
```

**SHELXD will separate space groups the merged intensities could not.** That sweep's report named a
body-centred cubic pair as indistinguishable, so SHELXC and SHELXD were run once per candidate —
same reflections, same `FIND`, only `SPAG` different. One gave `CC 37.93 / CC(weak) 14.05 /
CFOM 51.98` and the other `CC 46.76 / CC(weak) 22.61 / CFOM 69.37`. The substructure is where the
screw axis shows itself, and the second group is the right one. This is the same handover as
Phaser's arrived at from the other side, and it is worth doing whenever `SPACE_GROUP_ALTERNATIVES`
is not `NONE` — SHELXD takes seconds, and the pair of runs costs less than reprocessing anything.

**SHELXE decides the hand**, and says so. Run it twice, `-i` inverting the substructure. `-s` is the
solvent fraction, `-h` says the substructure atoms belong to the native structure, as sulfur does,
and `-a` turns on autotracing, which is what actually makes the two hands separate. The two runs
write `sad.pdb` and `sad_i.pdb`, so they can share a directory:

```
shelxe sad sad_fa -h -s0.62 -m20 -a15 -q
shelxe sad sad_fa -h -s0.62 -m20 -a15 -q -i
```

At 63 % solvent the two hands came out at 42.93 % and 15.28 % for the autotrace CC against the
native data — pseudo-free CC 66.49 against 37.12, map contrast 0.87 against 0.44, 215 traced atoms
— which is a solved structure, from `myrun.hkl` and nothing else. Where the group is one of the 22
that come in enantiomorphic pairs, SHELXE makes the group change itself: the inverted run prints
`** Space group converted to enantiomorph **` and writes the changed group into the `CRYST1` of its
traced model, so the answer is readable off the output file the same way it is off Phaser's.

**A negative result, for calibration.** A tetragonal dataset at the same wavelength with the same
kind of substructure, but 87 % complete at multiplicity 20 rather than 95 % at 30, gave a plausible
SHELXD `CFOM 47.62` and then failed at the hand: 15.33 % against 15.60 % autotrace CC, map contrast
0.33 either way. That is not a discrimination and it is not a solution. Nothing about the file was
the limit — the anomalous signal SHELXC measured on it was real, `<d"/σ>` reaching 4.2 — so the
reading is that sulfur phasing wants the completeness and the multiplicity, and a `.hkl` from a
sweep that does not have them will get this far and no further.

## Comparing the geometry with XDS

Every run logs the detector geometry a second time in **XDS's convention**, so it can be read
straight across against the `IDXREF.LP` / `CORRECT.LP` of an XDS run on the same data:

```
XDS convention: ORGX= 1091.00 ORGY= 1137.00 DETECTOR_DISTANCE= 75.0000
XDS convention: DIRECTION_OF_DETECTOR_X-AXIS= 1.000000 0.000000 0.000000
XDS convention: DIRECTION_OF_DETECTOR_Y-AXIS= 0.000000 1.000000 0.000000
XDS convention: INCIDENT_BEAM_DIRECTION= 0 0 1 X-RAY_WAVELENGTH= 1.000000 QX= QY= 0.075000
XDS convention: ROTATION_AXIS= -1.000000 0.000000 0.000000
```

XDS is never given this geometry — the [XDS plugin](SOFTWARE_INTEGRATION.md) supplies image data
only, and XDS refines its own from `XDS.INP` — which is what makes the comparison worth having. The
two laboratory frames coincide (x along increasing detector column, y along increasing row, z along
the beam), so the numbers are directly comparable, and a tilt appears as the two detector axis
vectors rather than as angles, which is how XDS reports it after refinement. Two things to keep in
mind: **`ORGX`/`ORGY` are 1-based**, because XDS counts pixels from 1 and Jungfraujoch from 0; and
they are the **PONI**, the same quantity Jungfraujoch's beam centre is — so no correction is needed —
but not the direct beam once the detector is tilted (see above).