Calculation of Delta-chi
Queen's University Protein Function Discovery
and Department of Biomedical and Molecular Sciences
Molecular Modelling and Crystallographic Computing Facility
Crystallography and Modelling:

Calculation of Delta-Chi (optimal chi offset for oscillation data collection)

In order to collect a complete data set, the crystal should be positioned, such that the cusp (the angular region of reciprocal space that does not cross the Ewald sphere during rotation of the crystal) does not lie too close to a crystallographic axis. After taking a single frame of data, the program strategy can be run to predict the optimum oscillation (phi) range and starting point.

If strategy suggests that complete data cannot be obtained, even with a 360-degree rotation of the crystal, then look at the values for rotz and roty. It is likely that they are too close to 0 (zero), or too close to 90 degrees if the axis defined in the spindle command in the denzo integration input file is not really aligned along the spindle. In the latter case it might be useful to use xdisp to determine which axis is closest to the spindle axis (pick spot predictions in the zoom window to see the reflection indices).

The following calculation is an attempt to define what the minimum offset of the crystal should be from the fully aligned position. If the value for delta-chi is less than the ideal calculated chi value, which is determined only by the maximum resolution, then it would be advisable to try to adjust the arcs of the goniometer head in order to tilt the crystal away from its alignment.

Chi is calculated using the formula:

chi = sin-1(lambda/(2*res))

Delta-Chi is calculated using the formula:

dchi = cos-1(cos(roty)*cos(rotz)), where
rotz = the rotation around the beam axis, and
roty = the rotation around the vertical axis, using the denzo notation.
In the case of the R-Axes (sic), the vertical axis is actually horizontal.
Enter values below (default wavelength is for copper Kalpha):
wavelength Å resolution Å
rotz degrees roty degrees

Chi degrees Delta-Chi: degrees
To Do Additional Calculations: Change the appropriate values above and press CALCULATE again.

Last revised: Tuesday, 16-Sep-2003 10:34:49 EDT