Problems

This is a collection of problems and their solution.

XDS crashes

XDS should never crash (it it terminates with an error message, this does not count as crash). If it does, it is either a bug in the program which should be brought to the attention of Wolfgang Kabsch or Kay Diederichs, and will be fixed, or it is a problem with your computer.

If it crashes for the second reason, there are three things to try:

• increase the stack limit of your shell, e.g. (if using csh or tcsh) with

limit stacksize 102400 

or (in case of bash)

ulimit -s 102400 

The numbers above mean a 10-fold increase over the default, and should be enough. I've found this to be necessary for unusually large frames (32 MB).

• if you used xds when it crashes, try xds_par, and vice versa. xds_par uses OpenMP for parallelization, which adds complexity.
• if the error message is

xds: Exec format error. Wrong Architecture.

make sure to use the 32bit version of XDS instead - you were trying to run the 64bit version on a 32bit operating system.

IDXREF ends with message

 !!! ERROR !!! SOLUTION IS INACCURATE
AUTOMATIC DATA PROCESSING STOPPED. AS THE CRITERIA FOR A GOOD
SOLUTION ARE RATHER STRICT, YOU MAY CHOOSE TO CONTINUE DATA
PROCESSING AFTER CHANGING THE "JOB="-CARD IN "XDS.INP" TO
"JOB= DEFPIX INTEGRATE CORRECT".
IF THE BEST SOLUTION IS REALLY NONSENSE YOU SHOULD FIRST HAVE
A LOOK AT THE ASCII-FILE "SPOT.XDS". THIS FILE CONTAINS THE
INITIAL SPOT LIST SORTED IN DECREASING SPOT INTENSITY. SPOTS
NEAR THE END OF THE FILE MAY BE ARTEFACTS AND SHOULD BE ERASED.
ALTERNATIVELY YOU MAY TRY DIFFERENT VALUES FOR "INDEX_ORIGIN"
AS SUGGESTED IN THE ABOVE LISTING.
IF THE CRYSTAL HAS SLIPPED AT THE BEGINNING OF DATA COLLECTION
YOU MAY CHOOSE TO SKIP SOME OF THE FIRST FRAMES BY CHANGING
THE "DATA_RANGE=" IN FILE "XDS.INP" AND START ALL OVER AGAIN.

Well, this is printed out for you to actually read, and take action accordingly. In many cases you just change the JOBS - line in XDS.INP to read

JOB= DEFPIX INTEGRATE CORRECT

and then continue to run XDS. In other cases you change other keywords in XDS.INP. But in any case this is an important alert that should make you check the correctness of the parameters that describe the data collection (X-RAY_WAVELENGTH, DETECTOR_DISTANCE, ORGX, ORGY, OSCILLATION_RANGE, NAME_TEMPLATE_OF_DATA_FRAMES).

Furthermore, you should definitively check FRAME.cbf after the XDS run (using XDS-viewer or adxv), to make sure that the predictions match the observed reflections.

IDXREF does not show the expected lattice

By specifying the spacegroup and unitcell, you tell XDS that it should index based on those reflections that match that spacegroup and unitcell. In a sense, you _force_ that spacegroup and cell. This will discard other (spurious?) reflections, and usually leads to a clean list of Bravais lattice possibilities. If the data reduction nevertheless fails (in terms of bad R-factors and ISa in the CORRECT step), then chances are that you specified some parameter wrongly, or not accurate enough (ORGY and ORGY are the most likely candidates), or that the crystal does not match your idea about its spacegroup and unit cell. Actually the latter happens pretty frequently (which is why it is always the safest way to collect 180° of spindle rotation unless you know your crystals very well; this is also a good strategy in other respects).

If you tell XDS that you do not to know the spacegroup (SPACE_GROUP_NUMBER= 0), then IDXREF takes all observed reflections into account. By design, the spacegroup decision is then postponed until the CORRECT step, or rather to a run of pointless after CORRECT, and it may be not vital to closely inspect IDXREF.LP, because CORRECT.LP has basically the same information, plus more.

none of the lattices in IDXREF.LP (except aP) has a good QUALITY OF FIT

It is a good idea to use many frames (e.g. the first half of the DATA_RANGE, as it is done in the generate_XDS.INP script) for SPOT_RANGE in the COLSPOT and IDXREF steps.

It is entirely possible to run COLSPOT for the full DATA_RANGE, and to try sub-ranges in IDXREF - this means COLSPOT has to be run only once, and the JOBS= line has e.g.

JOBS= IDXREF 

only. Maybe some sub-range gives a clear answer. In that case it may be useful to use

REFINE(INTEGRATE)=! AXIS BEAM ORIENTATION CELL DISTANCE

because otherwise the spurious reflections in the other sub-ranges will probably disturb the on-the-fly refinement of parameters.

I've seen datasets where each reflection had a satellite associated with the main reflection, but separate from it. In such a case it helps to use

MINIMUM_NUMBER_OF_PIXELS_IN_A_SPOT= 12 ! re-run COLSPOT after changing the parameter !

thus doubling (w.r.t. the default of 6) the required spotsize that makes a reflection be used for indexing. In the latest case this made it possible to index cleanly on a single frame (which is actually not uncommon).

A final possibility: your crystal may really be triclinic - hopefully you collected 180° of data, or even a bit more than that.